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>

drivers/gpu/drm/amd/pm/swsmu/smu13/aldebaran_ppt.c

@@ -1606,6 +1606,7 @@ out_unlock:
 	mutex_unlock(&smu->metrics_lock);
 
 	adev->unique_id = ((uint64_t)upper32 << 32) | lower32;
+	if (adev->serial[0] == '\0')
 		sprintf(adev->serial, "%016llx", adev->unique_id);
 }