drm/amdgpu: refine cz uvd clock gate logic.

sw clockgate was used on uvd6.0.
when uvd is idle, we gate the uvd clock.
when decode, we ungate the uvd clock.

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