drm/mgag200: Set cursor scanout address to correct BO

The hardware requires the correct memory address of the buffer. Currently
the same BO's address is programmed unconditionally, so only every second
cursor update actually becomes visible.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Fixes: 94dc57b1 ("drm/mgag200: Rewrite cursor handling")
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190723075425.24028-3-tzimmermann@suse.de