drm/amdgpu: disable gfxoff if VCN is busy

Toggle on/off gfxoff during video playback to fix gpu hang.

v2: change sequence to be more compatible with original code.

Signed-off-by: Jiansong Chen <Jiansong.Chen@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>