amdgpu/gmc_v9: save/restore sdpif regs during S3

fixes S3 issue with IOMMU + S/G  enabled @ 64M VRAM.

Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org