i965: Remove the forced lack of caching for renderbuffer surface state.

i965: Remove the forced lack of caching for renderbuffer surface state.

This snuck in with the multi-draw-buffers commit, and is a major penalty
to performance.  It doesn't appear to be required, as the only dependency
the surface BO has is on the state key (and if there's some other dependency,
it should just be in the key).

This brings openarena performance up to almost 2% faster than Mesa 7.4.