drm/amdgpu: add uvd enc ring test

Add UVD encode ring test functions. And enable UVD encode ring test
during UVD encode hardware initialization.

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>