drm/amdgpu/psp: add check sOS sign

Confirm if sys driver and sOS are already been loaded through sOS
sign register, skip loading sys driver and sOS if finding the sign.

Signed-off-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>