staging: xgifb: XGINew_GetXG21Sense(): eliminate video BIOS access

Remove video BIOS access from the routine, use the flag instead which
is initialized according to the BIOS data.

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