Redefine MESA_FORMAT as PIPE_FORMATs
Here we go: finally, unification of our format enum values. This is includes the commits from !1885 (merged), which was preparing for this. I haven't moved any util helpers yet, which is the end goal, but hopefully deleting 1000 lines of copy-and-paste code is a good enough justification for this work.
Clean piglit results on i965.