drm/virtio: Fix missed dmabuf unpinning in error path of prepare_fb()

Correct error handling in prepare_fb() to fix leaking resources when
error happens.

Fixes: 4a696a2 ("drm/virtio: Add prepare and cleanup routines for imported dmabuf obj")
Cc: <stable@vger.kernel.org> # v6.14+
Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://lore.kernel.org/r/20250401123842.2232205-2-dmitry.osipenko@collabora.com

Author: digetx

drivers/gpu/drm/virtio/virtgpu_plane.c

@@ -322,12 +322,6 @@ static int virtio_gpu_plane_prepare_fb(struct drm_plane *plane,
 		return 0;
 
 	obj = new_state->fb->obj[0];
-	if (obj->import_attach) {
-		ret = virtio_gpu_prepare_imported_obj(plane, new_state, obj);
-		if (ret)
-			return ret;
-	}
-
 	if (bo->dumb || obj->import_attach) {
 		vgplane_st->fence = virtio_gpu_fence_alloc(vgdev,
 						     vgdev->fence_drv.context,
@@ -336,7 +330,21 @@ static int virtio_gpu_plane_prepare_fb(struct drm_plane *plane,
 			return -ENOMEM;
 	}
 
+	if (obj->import_attach) {
+		ret = virtio_gpu_prepare_imported_obj(plane, new_state, obj);
+		if (ret)
+			goto err_fence;
+	}
+
 	return 0;
+
+err_fence:
+	if (vgplane_st->fence) {
+		dma_fence_put(&vgplane_st->fence->f);
+		vgplane_st->fence = NULL;
+	}
+
+	return ret;
 }
 
 static void virtio_gpu_cleanup_imported_obj(struct drm_gem_object *obj)