ct_driver: define XMODE_'s if not defined yet

Traditionally they've been defined in Xorg server headers, but since this driver seems to be the only consumer, it makes sense moving them here. (and later drop them from xorg headers)

Explicitly guarding it, so it also works with xorg headers version still carrying those symbols.

Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net