staging: xgifb: refactor XGI_BridgeIsOn()

Refactor XGI_BridgeIsOn(). IF_DEF_LVDS bit is not valid for < XG20 chips.
Also we can make the function static by moving it to vb_init.

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