staging: xgifb: delete incorrect I/O mapping

If the PCI device was disabled when the probe() routine started, the
driver will create 256 MB video memory mapping which is never used or
properly released. It's also unsafe as the size is incorrect for many
video cards. Deleting it also allows eliminating XGIvga_enable global
variable.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>