drm: use new iterator in drm_gem_plane_helper_prepare_fb v3

Makes the handling a bit more complex, but avoids the use of
dma_resv_get_excl_unlocked().

v2: improve coding and documentation
v3: adjust the TODO comment as suggested by Daniel
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005113742.1101-25-christian.koenig@amd.com

drivers/gpu/drm/drm_gem_atomic_helper.c

@@ -143,6 +143,7 @@
  */
 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;
 
@@ -150,9 +151,18 @@ int drm_gem_plane_helper_prepare_fb(struct drm_plane *plane, struct drm_plane_st
 		return 0;
 
 	obj = drm_gem_fb_get_obj(state->fb, 0);
-	fence = dma_resv_get_excl_unlocked(obj->resv);
-	drm_atomic_set_fence_for_plane(state, fence);
+	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);
 
+	drm_atomic_set_fence_for_plane(state, fence);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(drm_gem_plane_helper_prepare_fb);