drm/amdgpu: save/restore part of xcp drm_device fields

Redirect xcp allocated drm_device::rdev/pdev/driver with
amdgpu pci_device/drm_device setting. They need be saved
before redirect and restored after unregister xcp drm_device.

-v2: fix warning discarded-qualifiers

Signed-off-by: James Zhu <James.Zhu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>