drm/bridge: Document the drm_encoder.bridge_chain field as private

The drm_encoder.bridge_chain is not meant to be touched manually by
drivers. Make this clear in the documentation.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226112514.12455-5-laurent.pinchart@ideasonboard.com

include/drm/drm_encoder.h

@@ -174,7 +174,8 @@ struct drm_encoder {
 	struct drm_crtc *crtc;
 
 	/**
-	 * @bridge_chain: Bridges attached to this encoder.
+	 * @bridge_chain: Bridges attached to this encoder. Drivers shall not
+	 * access this field directly.
 	 */
 	struct list_head bridge_chain;