staging: xgifb: delete duplicated default video mode setup code

The code for determining the default video mode is unnecessarily
duplicated in XGIfb_search_mode() and XGIfb_search_vesamode() functions.

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