drm/amdgpu: Only check for S0ix if AMD_PMC is configured

The S0ix check only makes sense if the AMD PMC driver is
present.  We need to use the legacy S3 pathes when the
PMC driver is not present.

Reviewed-by: Prike Liang <Prike.Liang@amd.com>
Reviewed-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>