staging/xgifb: fix display on XGI Volari Z11m cards

Image on Z11m cards was totally garbled due to wrong memory being
selected. Add a special handling for Z11m cards. Tested on PCIe Z11 and
Z11m cards.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>