R9 285 tonga since

commit afbeb2d0961b2139bcf6553a710e6a8ae5d09d34
Author: Sylvia Tsai <sylvia.tsai@amd.com>
Date:   Tue Apr 11 15:15:28 2017 -0400

    drm/amd/display: Parse scanline registers

    They could differ between ASIC generations

    Change-Id: Ia352ae206273fe3a025579554cae4e3711a26fcc

on agd5f drm staging 4.9/11 there is a regression with full screen video.

mpv --vo=vaapi/vdpau/opengl all the same, dropping frames, or playing slowmo
with --framedrop=no

Windowed playback is OK, testing with 60fps vid on 60Hz DVI-D display.

Forcing clocks high does not help.