staging/xgifb: Include sis headers

This patch includes the headers of the sis driver and reorders some
includes.

Since the xgi driver used to redefine a lot of stuff from the sis
driver, we can simply include the headers of the sis driver itself, so
we can remove duplicated stuff later on.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>