We only need the lock for:
1. Rummaging through the cache
2. Allocating VMA

We don't need it for alloc_fresh_bo(), which does GEM_CREATE, and also
SET_DOMAIN to allocate the underlying pages.  The idea behind calling
SET_DOMAIN was to avoid a lock in the kernel while allocating pages,
now we avoid our own global lock as well.

We do have to re-lock around VMA.  Hopefully this shouldn't happen too
much in practice because we'll find a cached BO in the right memzone
and not have to reallocate it.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>