drm/bridge: Add the drm_bridge_get_prev_bridge() helper

The drm_bridge_get_prev_bridge() helper will be useful for bridge
drivers that want to do bus format negotiation with their neighbours.
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191203141515.3597631-7-boris.brezillon@collabora.com

include/drm/drm_bridge.h

@@ -426,6 +426,22 @@ drm_bridge_get_next_bridge(struct drm_bridge *bridge)
 	return list_next_entry(bridge, chain_node);
 }
 
+/**
+ * drm_bridge_get_prev_bridge() - Get the previous bridge in the chain
+ * @bridge: bridge object
+ *
+ * RETURNS:
+ * the previous bridge in the chain, or NULL if @bridge is the first.
+ */
+static inline struct drm_bridge *
+drm_bridge_get_prev_bridge(struct drm_bridge *bridge)
+{
+	if (list_is_first(&bridge->chain_node, &bridge->encoder->bridge_chain))
+		return NULL;
+
+	return list_prev_entry(bridge, chain_node);
+}
+
 /**
  * drm_bridge_chain_get_first_bridge() - Get the first bridge in the chain
  * @encoder: encoder object