Performance: Load all displays at once in panels_x_list_page

While panels_page_list_page and panels_mini_list_page do make use of the _load_all functions to retrieve the panels, they retrieve the appropriate displays one by one per panel. This adds up to one additional db query per panel.

Enclosed patch modifies the two functions to bulk-retrieve the displays of the given panels. This saves time if the list is long and doesn't do any harm if there's only one or two entry (the bulk query takes about the same time as each single query does).

Has been tested, except for programmatic panels.