WorkspacesDisplay: update the primary monitor index too

The primary monitor index might change so we need to update it too.

https://bugzilla.gnome.org/show_bug.cgi?id=743993

js/ui/workspacesView.js

@@ -538,6 +538,7 @@ const WorkspacesDisplay = new Lang.Class({
         for (let i = 0; i < this._workspacesViews.length; i++)
             this._workspacesViews[i].destroy();
 
+        this._primaryIndex = Main.layoutManager.primaryIndex;
         this._workspacesViews = [];
         let monitors = Main.layoutManager.monitors;
         for (let i = 0; i < monitors.length; i++) {