drm/amdgpu/uvd: UVD entity initialization relys on ring initialization

Entity init should after ring init, as the entity's sched_rq's initialization
is in ring init.

SWDEV-161495

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