https://bugs.freedesktop.org/show_bug.cgi?id=110865
--- Comment #12 from Dieter Nützel Dieter@nuetzel-hh.de --- (In reply to Alex Deucher from comment #9)
Created attachment 144983 [details] [review] fix DC code
Can you try applying both of these patches? Assuming both of your monitors have the same timing this might work.
Didn't apply on amd-staging-drm-next, too.