Use u_lockless_array for per-context BOs
We would like to store per-context, per-BO information. Ken's iris-ula branch adds u_lockless_array and an iris_ctx_bo structure, which can store these things.
One example is bo->index, which is the slot in a context's validation list. In i965, we cheat and just store one globally, and sanity check it. In Iris, that doesn't make much sense, since a single GL context will have 2-3 GEM contexts (render, compute, possibly blorp in the future). So we really want it to be per-context.