drm/amdgpu: Sienna_Cichlid don't enable SMU for SRIOV

SMU firmware already been loaded from host, don't enable it for now.
May need to re-work it if we want to enable the SMU for guest in the future.

Signed-off-by: shaoyunl <shaoyun.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>