appDisplay: Set 3x3 grid mode for folders

Folders have a fixed 3x3 grid, given that folders themselves
have a fixed size. Make the code correspond to this invariant.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2335>

js/ui/appDisplay.js

@@ -1974,6 +1974,13 @@ class FolderGrid extends IconGrid.IconGrid {
             page_halign: Clutter.ActorAlign.CENTER,
             page_valign: Clutter.ActorAlign.CENTER,
         });
+
+        this.setGridModes([
+            {
+                rows: 3,
+                columns: 3,
+            },
+        ]);
     }
 });