Make driver work again on current xorg-server + some DMA cache related improvements

Make driver work again on current xorg-server + some DMA cacheflush related improvements for SGI Indigo2 Impact10000 (IP28), with proper fallback to PIO mode. Most code is a forward port of Peter Fuersts old patch.

Signed-off-by: Barnabás Virágh cyborgyn@gmail.com