xwayland: Check for X11 unix directory only once

The function choose_xdisplay() calls open_display_sockets() which calls
ensure_x11_unix_dir().

We don't need to do that from within the loop though, as the directory
/tmp/.X11-unix is the same regardless of the display number.

Move the call to ensure_x11_unix_dir() from open_display_sockets() to
choose_xdisplay() prior to enter the display loop.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1669>

src/wayland/meta-xwayland.c

@@ -649,9 +649,6 @@ open_display_sockets (MetaXWaylandManager  *manager,
 {
   int abstract_fd, unix_fd;
 
-  if (!ensure_x11_unix_dir (error))
-    return FALSE;
-
   abstract_fd = bind_to_abstract_socket (display_index, fatal, error);
   if (abstract_fd < 0)
     return FALSE;
@@ -684,6 +681,9 @@ choose_xdisplay (MetaXWaylandManager     *manager,
   else if (g_getenv ("RUNNING_UNDER_GDM"))
     display = 1024;
 
+  if (!ensure_x11_unix_dir (error))
+    return FALSE;
+
   do
     {
       g_autoptr (GError) local_error = NULL;