drm/amd/powerplay: partial revert commit 01b0e7fb1.

when uvd is idle, we gate uvd clock.
and uvd is busy, we ungate uvd clock.

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