drm/amdgpu: implement UVD VM mode for Stoney v2

Starting with Stoney we support running UVD in VM mode as well.

v2: rebased, only enable on Polaris for now.

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