drm/bridge: Fix a NULL pointer dereference in drm_atomic_bridge_chain_check()

drm_atomic_bridge_chain_check() callers can pass a NULL bridge. Let's
bail out before dereferencing the bridge pointer when that happens.
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: b86d8955 ("drm/bridge: Add an ->atomic_check() hook")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200107113031.435604-1-boris.brezillon@collabora.com

drivers/gpu/drm/drm_bridge.c

@@ -938,15 +938,19 @@ int drm_atomic_bridge_chain_check(struct drm_bridge *bridge,
 				  struct drm_connector_state *conn_state)
 {
 	struct drm_connector *conn = conn_state->connector;
-	struct drm_encoder *encoder = bridge->encoder;
+	struct drm_encoder *encoder;
 	struct drm_bridge *iter;
 	int ret;
 
+	if (!bridge)
+		return 0;
+
 	ret = drm_atomic_bridge_chain_select_bus_fmts(bridge, crtc_state,
 						      conn_state);
 	if (ret)
 		return ret;
 
+	encoder = bridge->encoder;
 	list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) {
 		int ret;