GEM v. TTM

The early benchmarking is kind of unfortunate -- we just started writing this code, and have
needed to spend more time on correctness than performance so far.  I've still got issues on
the 965 to resolve.  But keithp put in changes last week that got another 16% performance
improvement on my 945 system with GEM, I think we've got room for improvement on 915-class
still, and I know there's serious low-hanging fruit in 965 with GEM.

Right now, though, I care most about getting a solid user API that we can feel comfortable
putting into the kernel and maintaining for the forseeable future.  The only issue I have with
GEM API at the moment is the cache domain setting being general as opposed to driver-specific
API.  So far when we try to make a general API describing some bit of hardware state with an
N-bit field, it seems some other driver developer says he needs about 4N bits.