make ttm_device_prepare_hibernation() return early

kettenis reports the ttm_tm_swapout() change is not enough to fix
hibernate with amdgpu when X is running.

The call to ttm_device_prepare_hibernation()/ttm_tm_swapout() in the
hibernate path was added in a linux commit from 2025:
924dda024f3bea64be5f3ac067a075e466739dc9
drm/amdgpu: move GTT to shmem after eviction for hibernation

The linux reason for doing this (server cards with lots of VRAM) isn't
something that concerns us.

ok kettenis@

sys/dev/pci/drm/ttm/ttm_device.c

@@ -130,6 +130,8 @@ out:
  */
 int ttm_device_prepare_hibernation(struct ttm_device *bdev)
 {
+	return 0;
+#ifdef notyet
 	struct ttm_operation_ctx ctx = {
 		.interruptible = false,
 		.no_wait_gpu = false,
@@ -140,6 +142,7 @@ int ttm_device_prepare_hibernation(struct ttm_device *bdev)
 		ret = ttm_device_swapout(bdev, &ctx, GFP_KERNEL);
 	} while (ret > 0);
 	return ret;
+#endif
 }
 EXPORT_SYMBOL(ttm_device_prepare_hibernation);