alloc fence for imported bo when prepare fb

Signed-off-by: Xue, Bosheng <bosheng.xue@intel.com>

Author: bosheng1

drivers/gpu/drm/virtio/virtgpu_plane.c

@@ -634,7 +634,7 @@ static int virtio_gpu_plane_prepare_fb(struct drm_plane *plane,
 	if (!bo || (plane->type == DRM_PLANE_TYPE_PRIMARY && !bo->guest_blob))
 		return 0;
 
-	if (bo->dumb) {
+	if (bo->dumb || vgfb->base.obj[0]->import_attach) {
 		vgplane_st->fence = virtio_gpu_fence_alloc(vgdev,
 						     vgdev->fence_drv.context,
 						     0);