staging: xgifb: move video BIOS handling completely to ReadVBIOSTablData()

We need the video BIOS only on XG21 and only during the init, so let's
do everything at the same place.

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