drm/amdgpu/gmc8: skip MC ucode loading on SR-IOV capable boards

VBIOS does this for us in asic_init.

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