A complete shoot in the dark, but please try to disable semaphores by just returning false from uvd_v2_2_semaphore_emit() in drivers/gpu/drm/radeon/uvd_v2_2.c. Otherwise I'm running out of ideas what that could be. The only other explanation I have is that the added load of UVD+displaying at a certain resolution is crashing the GFX block. But I have no idea why that should happen.