staging: xgifb: correct the default mode comment

According to the comment, the default mode should be 800x600x8 while
the index points actually to 800x600x16. Change the comment and move
the definition instead of changing the current behaviour.

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