drm/radeon: fix fence locking in the pageflip callback

We need to hold bdev->fence_lock while grabbing a reference to
the fence, to prevent concurrent clearing/changing of the
ttm_bo->sync_obj field.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_display.c

@@ -378,8 +378,12 @@ static int radeon_crtc_page_flip(struct drm_crtc *crtc,
 	work->old_rbo = rbo;
 	obj = new_radeon_fb->obj;
 	rbo = gem_to_radeon_bo(obj);
+
+	spin_lock(&rbo->tbo.bdev->fence_lock);
 	if (rbo->tbo.sync_obj)
 		work->fence = radeon_fence_ref(rbo->tbo.sync_obj);
+	spin_unlock(&rbo->tbo.bdev->fence_lock);
+
 	INIT_WORK(&work->work, radeon_unpin_work_func);
 
 	/* We borrow the event spin lock for protecting unpin_work */