staging: xgifb: probe: delete checks for mode "none"

The driver does not allow selecting the mode "none", so no need to check
for that in probe.
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Reviewed-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>