The key problem was that the aperture size detection using the MSAC bit
doesn't work -- the bit appears to be set even when it shouldn't be. Linux
takes a different approach, testing for a bit of the GMADR (PCIR_BAR(2)) being
set. However, as I don't think that's a safe way to test aperture size, we
just allocate the resource and check its size. This also pointed out that
agp_generic_attach hadn't been allocating our aperture resource, which may
have caused problems in some cases.
Also corrected is a minor copy-and-pasteo in an error case.
PR: kern/103079
Submitted by: mnag
Tested on: i945GM, i915GM
MFC after: 2 weeks
30e14d656c