drm/amdgpu: implement harvesting support for UVD 7.2 (v3)

Properly handle cases where one or more instance of the IP
block may be harvested.

v2: make sure ip_num_rings is initialized amdgpu_queue_mgr.c
v3: rebase on Christian's UVD changes, drop unused var

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