At Linux 4.18.8 the HDMI monitor started to report "No Signal" although Xorg can see a screen when it's started.(There's no signal with or without X).
Using bisection, I found the change that caused this is commit to be 691f2d763d0731224439686ecf2d440df8fe910e:

+++ b/drivers/gpu/drm/amd/display/dc/bios/command_table.c
@@ -808,6 +808,24 @@ static enum bp_result transmitter_control_v1_5(
         * (=1: 8bpp, =1.25: 10bpp, =1.5:12bpp, =2: 16bpp)
         * LVDS mode: usPixelClock = pixel clock
         */
+       if  (cntl->signal == SIGNAL_TYPE_HDMI_TYPE_A) {
+               switch (cntl->color_depth) {
+               case COLOR_DEPTH_101010:
+                       params.usSymClock =
+                               cpu_to_le16((le16_to_cpu(params.usSymClock) * 30) / 24);
+                       break;
+               case COLOR_DEPTH_121212:
+                       params.usSymClock =
+                               cpu_to_le16((le16_to_cpu(params.usSymClock) * 36) / 24);
+                       break;
+               case COLOR_DEPTH_161616:
+                       params.usSymClock =
+                               cpu_to_le16((le16_to_cpu(params.usSymClock) * 48) / 24);
+                       break;
+               default:
+                       break;
+               }
+       }
 
        if (EXEC_BIOS_CMD_TABLE(UNIPHYTransmitterControl, params))
                result = BP_RESULT_OK;

Inserted printk stmts showed my monitor reports COLOR_DEPTH_121212. I suspect the clock speed is too high for the video card, so the code needs to check the card model before bumping the speed, but I don't know how to do that.
Here are all the details I an think of:
Video card (from lspci): VGA compatible controller:
Advanced Micro Devices, Inc. [AMD/ATI] Radeon R2 Graphics (rev ea)
Monitor (from /var/log/Xorg.0.log): Monitor name: DELL U2711
Ranges: V min: 49 V max: 86 Hz, H min: 29 H max: 113 kHz, PixClock max 285 MHz
Hardware name (from /var/log/debug):
HP HP Laptop 15-bw0xx/8330, BIOS F.11 07/05/2017