staging: xgifb: refactor XGI_GetLVDSData()

Refactor XGI_GetLVDSData() to avoid nesting and redundant ifs.

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