drm/radeon/dpm: use multiple UVD power states (v3)

Use the UVD handle information to determine which
which power states to select when using UVD.  For
example, decoding a single SD stream requires much
lower clocks than multiple HD streams.

v2: switch to a cleaner dpm/uvd interface
v3: change the uvd power state while streams
are active if need be

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