staging: xgifb: eliminate fbcon_XGI_fillrect()

fbcon_XGI_fillrect() implementation is trivial and can be replaced with
cfb_fillrect().

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