Comment # 7 on bug 1225147 from Takashi Iwai

It's a good news.  At least we're heading to the right direction.

I can backport the workaround patch to TW, but since the upstream got a
significant rewrite of the relevant code, let's check whether it covers your
problem at first.

I'm building another test kernel with two upstream backports:
2d5bb791e24f43b6b4231b7973009987bbcc9b06
  drm/amd/display: Implement update_planes_and_stream_v3 sequence
d62d5551dd615f9e488b13595d69b308cd019e16
  drm/amd/display: Backup and restore only on full updates

It's being built in OBS home:tiwai:bsc1225147 repo.  The package will appear at
  http://download.opensuse.org/repositories/home:/tiwai:/bsc1225147/standard/
Please give it a try later.

Meanwhile, you can join to the upstream gitlab.freedesktop.org issues mentioned
in comment 2, echoing that the revert helped, too.