uc_payment loads display_only line items into order.

The payment preview pane on the checkout page sets $order->line_items to the results of uc_order_load_line_items_display() - which seems contrary to the intention of 'display_only'. It also relies on the $order->line_items array not being refreshed via another checkout pane calling uc_order_load_line_items() -- if that happens, the display_only line items will not appear. Patch to follow regularizes this.