x11: Don't change mode if we are already in the correct mode If we are already in the desired mode, changing it is a no-op at best, and harmful at worst: on Xwayland, it sometimes happens that we disable the crtc and cannot re-enable it. Resolves: https://github.com/libsdl-org/SDL/issues/4630 Signed-off-by: Simon McVittie <smcv@collabora.com>