https://bugzilla.kernel.org/show_bug.cgi?id=54381
--- Comment #13 from Andrew Stubbs ams@codesourcery.com --- The "problem" code appears to be in drivers/gpu/drm/radeon/atombios_crtc.c:radeon_get_shared_nondp_ppll or possibly radeon_get_shared_dp_ppll.
I've tried making the clock matching code slightly more permissive, but no luck yet. I'm going to try a few more experiments.