st/nir: fix atomic lowering for gallium drivers

st/nir: fix atomic lowering for gallium drivers

i965 and gallium handle the atomic buffer index differently. It was
just by luck that the single piglit test for this was passing.

For gallium we use the atomic binding so that we match the handling
in st_bind_atomics().

On radeonsi this fixes the CTS test:
KHR-GL43.shader_storage_buffer_object.advanced-write-fragment

It also fixes tressfx hair rendering in Tomb Raider.

Reviewed-by: Marek Olšák  <marek.olsak@amd.com>