staging: xgifb: XGIfb_mode_rate_to_ddata: initialize ModeIdIndex properly

Initialize ModeIdIndex according to the selected video mode. Currently
index 0 is always used and wrong video mode data may be used.

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