iris: Destroy transfer slab after batches

iris: Destroy transfer slab after batches

Batches are going to have an uploader in the next commit, so destroying
batches will destroy uploaders, which will unmap transfers, which will
return things to the slab allocator.  So we need to reorder destroying
the slab allocator to the end to avoid crashing.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3802>