staging: xgifb: avoid direct references xgi_video_info

Avoid direct references to global xgi_video_info. This will help changing
the data from statically allocated to dynamically allocated.
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>