drm/atomic: Avoid connector to writeback_connector casts

Use container_of() instead of type casting so that it keeps working
even if base is moved inside the drm_writeback_connector struct.
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703075022.15138-2-boris.brezillon@bootlin.com

drivers/gpu/drm/drm_atomic.c

@@ -2428,6 +2428,7 @@ static int prepare_signaling(struct drm_device *dev,
 	}
 
 	for_each_new_connector_in_state(state, conn, conn_state, i) {
+		struct drm_writeback_connector *wb_conn;
 		struct drm_writeback_job *job;
 		struct drm_out_fence_state *f;
 		struct dma_fence *fence;
@@ -2451,7 +2452,8 @@ static int prepare_signaling(struct drm_device *dev,
 		f[*num_fences].out_fence_ptr = fence_ptr;
 		*fence_state = f;
 
-		fence = drm_writeback_get_out_fence((struct drm_writeback_connector *)conn);
+		wb_conn = drm_connector_to_writeback(conn);
+		fence = drm_writeback_get_out_fence(wb_conn);
 		if (!fence)
 			return -ENOMEM;
 

include/drm/drm_writeback.h

@@ -110,6 +110,12 @@ struct drm_writeback_job {
 	struct dma_fence *out_fence;
 };
 
+static inline struct drm_writeback_connector *
+drm_connector_to_writeback(struct drm_connector *connector)
+{
+	return container_of(connector, struct drm_writeback_connector, base);
+}
+
 int drm_writeback_connector_init(struct drm_device *dev,
 				 struct drm_writeback_connector *wb_connector,
 				 const struct drm_connector_funcs *con_funcs,