debxo: use initramfs-tools and xserver-xorg-video-geode from backports

This switches over to using backports.org for initramfs-tools and
xserver-xorg-video-geode.  Slowly, the other packages will make
their way to backports.org as well (until we no longer need
that annoying archive hack).

Signed-off-by: Andres Salomon <dilinger@collabora.co.uk>