staging: xgifb: make XGIfb_has_VB return boolean

This patch makes XGIfb_has_VB return boolean, since this function
only uses either one or zero as its return value.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>