AGP support for newer Intel chips (9xx)

[Kimura Fuyuki]

Here's a patch to add an AGP GART support for newer Intel chips. I've tested 
it with i945G + modular-xorg + several media players.

http://www.hadaly.org/fuyuki/agp-i915.patch

Note that I'm new to DF kernel and the patch is mostly a blind merge from 
FreeBSD.

On FreeBSD i845G support seems to be  accidentally dropped at r1.37.
 I recovered it in the patch above.

On DragonFly PCI_MAP_FIXUP option is required for i915 variants because 
AGP_APBASE and AGP_I915_MMADR points the same address. (?)
Some rework would be needed to eliminate redundant allocations.

Known bug: panic on detach. I don't know why the function is working on 
FreeBSD. (maybe not?)