staging: xgifb: eliminate IF_DEF_LVDS checks from vb_init

Inside vb_init, IF_DEF_LVDS tells only if the vbios was succesfully read
on XG21. Rearrange the code so that we don't need to set or care about
this flag.

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