no layout settings for subdisplays

When defining a 'Node nid' argument, and depending on the 'content types' checkboxes selected on the argument settings form, you get secondary tabs for each node under the 'Layout settings' and 'Content' tabs, to define type-specific behaviour.

The 'Layout settings' form under node-type subdisplays shows up incomplete (no 'per panel' style selectors). This is because the $display param of panels_edit_layout_settings() is empty. Looks like a typo / wrong copy-paste in panels_page_edit_layout_settings(), that fetches a $display but does not use it. Attached patch fixes this.