backends: Fix output cycling in non display-attached tablets

It would only allow to alternate between the logical monitors, we actually
want to return NULL here so it can cycle to the whole span of monitors.

https://bugzilla.gnome.org/show_bug.cgi?id=782032

src/backends/meta-input-settings.c

@@ -1660,7 +1660,7 @@ cycle_logical_monitors (MetaInputSettings   *settings,
       if (l->next)
         *next_logical_monitor = l->next->data;
       else
-        *next_logical_monitor = logical_monitors->data;
+        *next_logical_monitor = NULL;
     }
 
   return TRUE;