drm/nouveau/agp: add a quirk list to limit agp modes

Certain combinations of hardware can't actually support the maximum
detected speed. Add a quirk list that lists pairs of hostbridge/chip pci
ids and the mode that they should work with.

See https://bugs.freedesktop.org/show_bug.cgi?id=20341

Reported-by: Jason Detring <detringj@gmail.com>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>