drm/amdgpu: keep stolen memory for arct

Any dce register read back from arct is invalid. use hard code
stolen memory for arct until we validate the s3.

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