drm/amdgpu: Only overwrite serial if field is empty

On Aldebaran, the serial may be obtained from the FRU. Only overwrite
the serial with the unique_id if the serial is empty. This will support
printing serial numbers for mGPU devices where there are 2 unique_ids
for the 2 GPUs, but only one serial number for the board

Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>