This is where I'm up to :

git bisect start
# bad: [135c5504a600ff9b06e321694fbcac78a9530cd4] Merge tag
'drm-next-2018-06-06-1' of git://anongit.freedesktop.org/drm/drm
git bisect bad 135c5504a600ff9b06e321694fbcac78a9530cd4
# good: [c76f0b2cc2f1be1a8a20f0fe2c0f30919bc559fb] Merge tag
'drm-amdkfd-next-2018-05-14' of git://people.freedesktop.org/~gabbayo/linux
into drm-next
git bisect good c76f0b2cc2f1be1a8a20f0fe2c0f30919bc559fb
# good: [92400b8c8b42e53abb0fcb4ac75cb85d4177a891] Merge branch
'locking-core-for-linus' of
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
git bisect good 92400b8c8b42e53abb0fcb4ac75cb85d4177a891
# good: [07c4dd3435aa387d3b58f4e941dc516513f14507] Merge tag 'usb-4.18-rc1' of
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb
git bisect good 07c4dd3435aa387d3b58f4e941dc516513f14507
# bad: [cac18c82e0c5b39b69648942576dbd1d6f9d056e] drm/amdgpu: Specify vega20
uvd firmware
git bisect bad cac18c82e0c5b39b69648942576dbd1d6f9d056e
# bad: [cac18c82e0c5b39b69648942576dbd1d6f9d056e] drm/amdgpu: Specify vega20
uvd firmware
git bisect bad cac18c82e0c5b39b69648942576dbd1d6f9d056e
# bad: [cac18c82e0c5b39b69648942576dbd1d6f9d056e] drm/amdgpu: Specify vega20
uvd firmware
git bisect bad cac18c82e0c5b39b69648942576dbd1d6f9d056e
# bad: [cac18c82e0c5b39b69648942576dbd1d6f9d056e] drm/amdgpu: Specify vega20
uvd firmware
git bisect bad cac18c82e0c5b39b69648942576dbd1d6f9d056e
# skip: [7ab3fdde04218c4733e96712b651751c413d51c3] drm/amd/display: Update MST
edid property every time
git bisect skip 7ab3fdde04218c4733e96712b651751c413d51c3
# skip: [e930793280799e66c3197e2ee6e70b1129f8aa12] drm/amdgpu: add VEGAM pci
ids
git bisect skip e930793280799e66c3197e2ee6e70b1129f8aa12
# good: [53f071e19d566e7d0a4eada1bd8313a4cdb660a4] Merge drm/drm-next into
drm-intel-next-queued
git bisect good 53f071e19d566e7d0a4eada1bd8313a4cdb660a4
# good: [53f071e19d566e7d0a4eada1bd8313a4cdb660a4] Merge drm/drm-next into
drm-intel-next-queued
git bisect good 53f071e19d566e7d0a4eada1bd8313a4cdb660a4
# skip: [ba8f7ad0e5b25851299cd45a63b57d843e50b577] drm/amdgpu: add VEGAM UVD
firmware support
git bisect skip ba8f7ad0e5b25851299cd45a63b57d843e50b577
# skip: [d10fb4a6f382474025f326bf90ee3b64396486ea] drm/amd/pp: Change
pstate_clk frequency unit to 10KHz on Rv
git bisect skip d10fb4a6f382474025f326bf90ee3b64396486ea

I'll keep trying, hoping it ends before it is fixed by some other changes.