drm/amdgpu: skip VM inv eng assignment for mes ring

Statically allocated VM inv eng of gfxhub on sienna_cichlid is used up.
Also VM inv eng is no need for mes ring.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>