drm/radeon: use variable UVD clocks

Now that Christian fixed the performance problems with
the feedback buffer in mesa, we can enable variable UVD
clocks.  There are multiple UVD power states associated
with different types and numbers of streams.  This uses
the appropriate state based on that information rather
than always using the fastest UVD clocks which saves some
power.  One possible downside is that this may adversely
affect decode benchmarks since these power states target
specific playback requirements rather than maximum
performance.  If that becomes an issue, we can add a
sysfs attribute to force the max UVD state.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>