drm/radeon/dpm: add new callback for powergating UVD (v4)

Starting on CIK, multi-media blocks like UVD no longer
have special power state.  Rather they have their own
DPM implementation which adjusts their clocks dynamically
when active.  When they are not active, the blocks are
powergated to save power.

v2: add missing pm locks
v3: rebase on uvd state selection rework
v4: fix inverted logic typo noticed by Christian

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