[AGPGART] Numerous AMD64 gart driver cleanups.

[AGPGART] Numerous AMD64 gart driver cleanups.
From Andi Kleen.

- Fix the help text for the 8151 driver
- Fix the dependencies (must be compiled in when the IOMMU is in)
- Add __setup options for when the AGP driver is compiled in:
agp=off  agp=try_unsupported
Currently only supported for the K8 driver, the other drivers
would need fixes in their module init functions too.
- Add try_unsupported support for the K8 driver.
- Add some aperture sanity checking to the K8 driver.
There are unfortunately still BIOS around that get it wrong.
- Also try to read the aperture from the AGP bridge if it is bogus
in the Northbridge. Windows only looks into the bridge and some BIOS only
put the aperture there.
[These two changes are only useful for 32bit kernels. The 64bit kernel
checks this in aperture.c anyways, and fixes it. The 32bit kernel
cannot fix a complety broken aperture currently, but at least it will
not crash now]
- Clean up handling for multiple northbridges. The paths are the same
now for as for a single NB.
- Some other minor cleanups.