The following commit cause GPU lockup on CIK when starting X:

feebe91aa9a9d99d9ec157612a614fadb79beb99 is the first bad commit
commit feebe91aa9a9d99d9ec157612a614fadb79beb99
Author: Christian König <christian.koenig@amd.com>
Date:   Fri Feb 26 16:18:15 2016 +0100

    drm/amdgpu: apply gfx_v8 fixes to gfx_v7 as well
    
    We never ported that back to CIK, so we could run into VM faults here.
    
    Signed-off-by: Christian König <christian.koenig@amd.com>
    Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
    Cc: stable@vger.kernel.org

:040000 040000 1229f0ca17d5518c15d64c773303dd494b3c74a8 d31a4da20d5bec8c9a59608b86a928bd06e4cd0b M      drivers


When reverting this patch on kernel 4.5 final and Fedora's 4.6-rc0.git sources, no GPU lockup happens anymore.


Bisect history (Starting from 4.5-rc5):

git bisect start
# good: [81f70ba233d5f660e1ea5fe23260ee323af5d53a] Linux 4.5-rc5
git bisect good 81f70ba233d5f660e1ea5fe23260ee323af5d53a
# bad: [710d60cbf1b312a8075a2158cbfbbd9c66132dcc] Merge branch 'smp-hotplug-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
git bisect bad 710d60cbf1b312a8075a2158cbfbbd9c66132dcc
# bad: [7ae9c768e19ec8ce1b397a5c4abf88f9ee053e09] Merge tag 'pm+acpi-4.5-final' of git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm
git bisect bad 7ae9c768e19ec8ce1b397a5c4abf88f9ee053e09
# good: [f691b77b1fc491dae601631c8531a0a13e915466] Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs
git bisect good f691b77b1fc491dae601631c8531a0a13e915466
# bad: [848819c5447eb318a787467e8435a07c733e16c9] Merge tag 'imx-drm-fixes-2016-02-19' of git://git.pengutronix.de/git/pza/linux into drm-fixes
git bisect bad 848819c5447eb318a787467e8435a07c733e16c9
# bad: [e5322c54068162846fdbac0f0891cd95f37d4c4e] Merge branch 'for-linus2' of git://git.kernel.dk/linux-block
git bisect bad e5322c54068162846fdbac0f0891cd95f37d4c4e
# good: [da35825d9a091a7a1d5824c8468168e2658333ff] nvme: set queue limits for the admin queue
git bisect good da35825d9a091a7a1d5824c8468168e2658333ff
# bad: [26bae5e04c3570728787f809b13546a8169fdf95] Merge tag 'drm/tegra/for-4.5-rc7' of git://anongit.freedesktop.org/tegra/linux into drm-fixes
git bisect bad 26bae5e04c3570728787f809b13546a8169fdf95
# bad: [89913ea615754163ae9a066c9c1b95aa2a2f51b4] drm/amdgpu/cz: remove commented out call to enable vce pg
git bisect bad 89913ea615754163ae9a066c9c1b95aa2a2f51b4
# good: [4ea2efae0d117c5b5f44081bab941542d892e758] drm/amd/powerplay: send event to notify powerplay all modules are initialized.
git bisect good 4ea2efae0d117c5b5f44081bab941542d892e758
# bad: [9cac537332f5502c103415b25609548c276a09f8] drm/amdgpu/gfx8: specify which engine to wait before vm flush
git bisect bad 9cac537332f5502c103415b25609548c276a09f8
# bad: [feebe91aa9a9d99d9ec157612a614fadb79beb99] drm/amdgpu: apply gfx_v8 fixes to gfx_v7 as well
git bisect bad feebe91aa9a9d99d9ec157612a614fadb79beb99
# first bad commit: [feebe91aa9a9d99d9ec157612a614fadb79beb99] drm/amdgpu: apply gfx_v8 fixes to gfx_v7 as well