Replace '84' magic value by the X_MAX definition, and '1' by Y_MAX.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20220819153931.3147384-2-peter.maydell@linaro.org