drm/radeon/dpm: implement UVD powergating for CI

Disable the UVD block when not in use to save power.
The block is not actually powergated on CI, but we
switch between UVD DPM (where the uvd clocks are
adjusted on demand) and clocks off.

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