drm/amdgpu: add function to program pbb mode for sienna cichlid

Add function for sienna_cichlid to force PBB workload mode to zero by
checking whether there have SE been harvested.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>