In my Arch Linux installation, I compiled and installed new fresh kernel 4.19. 

I have non-standard hardware configuration where are: Radeon R7 360 (Bonaire), Radeon RX 480 (Ellesmere) and Radeon RX VEGA 56 (GFX900) graphics card.
If Radeon R7 360 was handled by Radeon driver (not AMDGPU) then system has been started properly without that errors. I solved temporarily trouble by supplying kernel parameter 'amdgpu.dc=0' (just disable DC). I have connetected monitor to Intel IGP (in the motherboard DVI port) and I am not using any Radeon graphics card to display screen.
After run and loading AMDGPU driver module kernel print errors (if all Radeon graphics cards are handled by AMDGPU driver and if no amdgpu.dc=0 parameter):

[   18.371385] WARNING: CPU: 1 PID: 289 at drivers/gpu/drm/drm_plane.c:182 drm_universal_plane_init+0x55b/0x5f0 [drm]
[   18.371386] Modules linked in: mousedev input_leds joydev led_class amdkfd intel_rapl x86_pkg_temp_thermal intel_powerclamp amd_iommu_v2 coretemp amdgpu(+) kvm_intel crct10dif_pclmul crc32_pclmul ghash_clmulni_intel pcbc aesni_intel aes_x86_64 radeon crypto_simd cryptd gpio_ich iTCO_wdt iTCO_vendor_support glue_helper intel_cstate chash gpu_sched snd_hda_codec_realtek ttm pcspkr intel_uncore snd_hda_codec_generic evdev intel_rapl_perf i2c_i801 snd_hda_codec_hdmi alx mei_me mei mdio ie31200_edac lpc_ich mac_hid snd_hda_intel snd_hda_codec snd_hda_core rtc_cmos snd_hwdep snd_pcm snd_timer snd soundcore pcc_cpufreq sg crypto_user ip_tables x_tables ext4 crc32c_generic crc16 mbcache jbd2 fscrypto sr_mod sd_mod cdrom uas usb_storage hid_generic ata_generic pata_acpi usbhid hid ata_piix xhci_pci libata
[   18.371446]  ehci_pci xhci_hcd ehci_hcd crc32c_intel scsi_mod usbcore usb_common i915 kvmgt vfio_mdev mdev vfio_iommu_type1 vfio kvm irqbypass intel_gtt i2c_algo_bit cec rc_core drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops drm agpgart
[   18.371470] CPU: 1 PID: 289 Comm: systemd-udevd Not tainted 4.19.0extratemps #1
[   18.371471] Hardware name: Gigabyte Technology Co., Ltd. To be filled by O.E.M./Z77-DS3H, BIOS F8 08/21/2012
[   18.371488] RIP: 0010:drm_universal_plane_init+0x55b/0x5f0 [drm]
[   18.371490] Code: e8 da e4 ff ff 48 8b b3 b8 04 00 00 31 d2 4c 89 ff e8 c9 e4 ff ff e9 08 fd ff ff 0f 0b c7 44 24 2c ea ff ff ff e9 06 fd ff ff <0f> 0b e9 2c fb ff ff bf 04 00 00 00 48 c7 c6 b0 f6 05 c0 e8 dd 21
[   18.371492] RSP: 0018:ffffafa001f4f938 EFLAGS: 00010246
[   18.371494] RAX: ffffffffc1206780 RBX: ffff9ffd092a1000 RCX: ffffffffc00e8ba0
[   18.371496] RDX: 0000000000000000 RSI: ffff9ffd0a84a800 RDI: ffff9ffd092a1000
[   18.371497] RBP: ffffafa001f4f9e8 R08: ffffffffc00e9120 R09: 0000000000000002
[   18.371498] R10: ffff9ffd1bbc74a0 R11: 0000000000000024 R12: 0000000000000002
[   18.371500] R13: 0000000000000000 R14: ffff9ffd0a84a800 R15: ffffffffc00e8ba0
[   18.371502] FS:  00007fe3e78b7480(0000) GS:ffff9ffd1f280000(0000) knlGS:0000000000000000
[   18.371504] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   18.371505] CR2: 00005610c22080b8 CR3: 0000000458860003 CR4: 00000000001606e0
[   18.371507] Call Trace:
[   18.371524]  ? drm_crtc_init+0x2a/0xb0 [drm_kms_helper]
[   18.371534]  drm_crtc_init+0x60/0xb0 [drm_kms_helper]
[   18.371631]  dce_v8_0_sw_init+0x17d/0x3b0 [amdgpu]
[   18.371723]  amdgpu_device_init.cold.14+0xd5f/0x1253 [amdgpu]
[   18.371730]  ? kmalloc_order+0x14/0x40
[   18.371799]  amdgpu_driver_load_kms+0x86/0x2c0 [amdgpu]
[   18.371816]  drm_dev_register+0x109/0x140 [drm]
[   18.371885]  amdgpu_pci_probe+0x13c/0x1c0 [amdgpu]
[   18.371892]  ? _raw_spin_unlock_irqrestore+0x20/0x40
[   18.371896]  local_pci_probe+0x41/0x90
[   18.371900]  pci_device_probe+0x189/0x1a0
[   18.371906]  really_probe+0x235/0x3a0
[   18.371910]  driver_probe_device+0xb3/0xf0
[   18.371913]  __driver_attach+0xdd/0x110
[   18.371916]  ? driver_probe_device+0xf0/0xf0
[   18.371919]  ? driver_probe_device+0xf0/0xf0
[   18.371921]  bus_for_each_dev+0x76/0xc0
[   18.371924]  bus_add_driver+0x152/0x230
[   18.371927]  ? 0xffffffffc15ca000
[   18.371930]  driver_register+0x6b/0xb0
[   18.371932]  ? 0xffffffffc15ca000
[   18.371936]  do_one_initcall+0x46/0x1f5
[   18.371942]  ? kmem_cache_alloc_trace+0x176/0x1d0
[   18.371945]  ? do_init_module+0x22/0x210
[   18.371947]  do_init_module+0x5a/0x210
[   18.371950]  load_module+0x2451/0x25f0
[   18.371957]  ? __se_sys_init_module+0x10a/0x170
[   18.371959]  __se_sys_init_module+0x10a/0x170
[   18.371965]  do_syscall_64+0x5b/0x170
[   18.371969]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   18.371971] RIP: 0033:0x7fe3eac4456e
[   18.371973] Code: 48 8b 0d f5 18 0c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 49 89 ca b8 af 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d c2 18 0c 00 f7 d8 64 89 01 48
[   18.371975] RSP: 002b:00007fffe22ba348 EFLAGS: 00000246 ORIG_RAX: 00000000000000af
[   18.371978] RAX: ffffffffffffffda RBX: 00005604f506e070 RCX: 00007fe3eac4456e
[   18.371979] RDX: 00005604f5067b00 RSI: 00000000006111e8 RDI: 00005604f5bb7760
[   18.371980] RBP: 00005604f5067b00 R08: 0000000000000007 R09: 0000000000000006
[   18.371982] R10: 00005604f504a010 R11: 0000000000000246 R12: 00005604f5bb7760
[   18.371983] R13: 00005604f5060be0 R14: 0000000000020000 R15: 00005604f506e070
[   18.371986] ---[ end trace e549c6398b2ff21b ]---

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These errors encountered many times in dmesg output.