bridge: Fix panel-bridge error return on !panel.

ERR_PTR() needs a negative errno argument.
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170615175423.17954-1-eric@anholt.net

drivers/gpu/drm/bridge/panel.c

@@ -161,7 +161,7 @@ struct drm_bridge *drm_panel_bridge_add(struct drm_panel *panel,
 	int ret;
 
 	if (!panel)
-		return ERR_PTR(EINVAL);
+		return ERR_PTR(-EINVAL);
 
 	panel_bridge = devm_kzalloc(panel->dev, sizeof(*panel_bridge),
 				    GFP_KERNEL);