drm/amdgpu: add uvd enc vm functions (v2)

Add UVD encode ring vm functions to handle frame ecoding.

v2: squash in warning fix (James)

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>