staging: xgifb: refactor XGI_SetXG21LVDSPara() and XGI_SetXG27LVDSPara()

The code is almost the same for XG21 and XG27, a single routine can
cover both.

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