a11y: Check display in `*grab_cell_focus()`

Calling the accessibility function `grab_focus()` on a `GtkCell` under
Wayland will cause the client to crash.

This is another case of `gdk_x11_get_server_time()` being called
regardless of the actual windowing backend used.

Closes: GNOME/gtk#1507
2 jobs for issue1507-bis in 6 minutes and 45 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build #170011

00:06:45

manual msys2-mingw32 #170012
win32 allowed to fail manual