Add probe line for the AMD 761 northbridge chip. At least it detects now,
seems to set up memory spaces correctly. This change actually did work for
me using -STABLE, XFree86 4.0.3 ~ some snapshot of DRI awhile back. I sent
mail to dfr to no avail, perhaps someone else would like to test it with
DRI.

Anyway, people have been nagging me about this change for awhile, so here's
the commit.