mesa: Make atomic lowering put atomics above SSBOs.

mesa: Make atomic lowering put atomics above SSBOs.

Gallium arbitrarily (it seems) put atomics below SSBOs, resulting in a
bunch of extra index management, and surprising shader code when you would
see your SSBOs up at index 16.  It makes a lot more sense to see atomics
converted to SSBOs appear as magic high numbers.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3240>