For some unknown reasons Nvidia NForce3 doesn't use the standard Hammer AGP architecture,
but requires set up of some shadow registers. This patch adds that to the K8 AGP driver.

Based on an old 2.4 patch from someone at Nvidia.

Also includes another bug fix for the K8 AGP handler, from Brad House.
We should not assume that there is only one northbridge in a Uniprocessor system.
Always flush all.

Also some minor cleanup.