drm/atomic-helper: support more than one write fence in drm_gem_plane_helper_prepare_fb

Use dma_resv_get_singleton() here to eventually get more than one write
fence as single fence.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220321135856.1331-13-christian.koenig@amd.com

drivers/gpu/drm/drm_gem_atomic_helper.c

@@ -143,25 +143,21 @@
  */
 int drm_gem_plane_helper_prepare_fb(struct drm_plane *plane, struct drm_plane_state *state)
 {
-	struct dma_resv_iter cursor;
 	struct drm_gem_object *obj;
 	struct dma_fence *fence;
+	int ret;
 
 	if (!state->fb)
 		return 0;
 
 	obj = drm_gem_fb_get_obj(state->fb, 0);
-	dma_resv_iter_begin(&cursor, obj->resv, false);
-	dma_resv_for_each_fence_unlocked(&cursor, fence) {
-		/* TODO: Currently there should be only one write fence, so this
-		 * here works fine. But drm_atomic_set_fence_for_plane() should
-		 * be changed to be able to handle more fences in general for
-		 * multiple BOs per fb anyway. */
-		dma_fence_get(fence);
-		break;
-	}
-	dma_resv_iter_end(&cursor);
+	ret = dma_resv_get_singleton(obj->resv, false, &fence);
+	if (ret)
+		return ret;
 
+	/* TODO: drm_atomic_set_fence_for_plane() should be changed to be able
+	 * to handle more fences in general for multiple BOs per fb.
+	 */
 	drm_atomic_set_fence_for_plane(state, fence);
 	return 0;
 }