drm/amd/powerplay: workaround for UVD clock issue

workaround issue that when uvd dpm disabled,
uvd clock remain high on polaris10. Manually turn
off the clocks.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Ken Wang <Qingqing.Wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>