drm/radeon: split out radeon_uvd_resume from uvd_v4_2_resume

For powergating, we just need to re-init the registers, there
is no need to restore the uvd BOs.  This just adds needless
work when powergating uvd for playback while the system is
on.  We only need to restore the uvd BOs on an actual resume
from suspend or when the driver loads.

This fixes multi-stream UVD playback on KB systems.

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