drm/amdgpu: add uvd enc rings

UVD 6.3 has two UVD encode rings.  Add the ring structures and initialize the hw ring buffers.
Currently only ASIC Polaris10/11/12 uses UVD6.3 encode engine on HEVC encoding.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-and-Tested-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>