I'm not completely confident that I marked every bad commit, but I got this:

c4120d55ffa44746584a8a1e5b00cb7eafc006ff is the first bad commit
commit c4120d55ffa44746584a8a1e5b00cb7eafc006ff
Author: Christian König <christian.koenig@amd.com>
Date:   Wed Jul 20 14:11:26 2016 +0200

    drm/amdgpu: use begin/end_use for UVD power/clock gating

    This fixes turning power and clock on when it is actually needed.

    Signed-off-by: Christian König <christian.koenig@amd.com>
    Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
    Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

:040000 040000 6dbab0f2ea26ecf48e88531eee491c2d0ebbc418
8dedcdeeb16e8a4f7a06a8934aaf912469be2171 M      drivers

Also the decodebin element uses vaapi to decode the video, so I may have
mischaracterized the bug in the first place that it's really an UVD problem.