drm/qxl: Use correct notify port address when creating cursor ring

The command ring and cursor ring use different notify port addresses
definition: QXL_IO_NOTIFY_CMD and QXL_IO_NOTIFY_CURSOR. However, in
qxl_device_init() we use QXL_IO_NOTIFY_CMD to create both command ring
and cursor ring. This doesn't cause any problems now, because QEMU's
behaviors on QXL_IO_NOTIFY_CMD and QXL_IO_NOTIFY_CURSOR are the same.
However, QEMU's behavior may be change in future, so let's fix it.

P.S.: In the X.org QXL driver, the notify port address of cursor ring
      is correct.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Cc: <stable@vger.kernel.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1585635488-17507-1-git-send-email-chenhc@lemote.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>