drm/bridge: Introduce drm_bridge_get_next_bridge()

And use it in drivers accessing the bridge->next field directly.
This is part of our attempt to make the bridge chain a double-linked list
based on the generic list helpers.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191203141515.3597631-3-boris.brezillon@collabora.com

drivers/gpu/drm/drm_encoder.c

@@ -171,7 +171,7 @@ void drm_encoder_cleanup(struct drm_encoder *encoder)
 		struct drm_bridge *next;
 
 		while (bridge) {
-			next = bridge->next;
+			next = drm_bridge_get_next_bridge(bridge);
 			drm_bridge_detach(bridge);
 			bridge = next;
 		}