sisfb: add subroutine for detecting XGI Z9

Z7 and Z9 have the same PCI ID, so additional checking is needed to
detect Z9. The method was "documented" in XGI's xgifb driver.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>