radeon: add support for reverse engineered xpress200m

The IGPGART setup code was traced using mmio-trace on fglrx by myself
and Phillip Ezolt <phillipezolt@gmail.com> on dri-devel.

This code doesn't let the 3D driver work properly as the card has no
vertex shader support.

Thanks to Matthew Garrett + Ubuntu for providing me some hardware to do this
work on.