Comment # 3 on bug 98376 from Michel Dänzer

Can you try a newer kernel? I'm afraid some fixes which might be relevant for
the "Pageflip completion event has impossible msc" messages have only landed in
4.9-rc though.

I'm not sure the issues with the second monitor are directly related to those
messages though. Might be related to this instead:

[  256.507250] [drm:radeon_dp_link_train [radeon]] *ERROR* channel eq failed: 5
tries
[  256.507269] [drm:radeon_dp_link_train [radeon]] *ERROR* channel eq failed
[  256.646139] [drm:radeon_dp_link_train [radeon]] *ERROR* channel eq failed: 5
tries
[  256.646156] [drm:radeon_dp_link_train [radeon]] *ERROR* channel eq failed