staging: xgifb: add a global variable for the default refresh rate

Add a global variable for the default refresh rate. This is done to
get rid of references to xgi_video_info before the probe routine, which
should allocate the xgi_video_info dynamically in the future.

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>