amdgpu/uvd: get ring reference from rq scheduler

base.sched may not be set for each instance and should not
be used for cases such as non-IB tests.

Fixes: 2320c9e6 ("drm/sched: memset() 'job' in drm_sched_job_init()")
Signed-off-by: David (Ming Qiang) Wu <David.Wu3@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>