st/mesa: add support for hw atomics to glsl->tgsi. (v5)

st/mesa: add support for hw atomics to glsl->tgsi. (v5)

This adds support for creating the hw atomic tgsi from
the glsl codepaths.

v2: drop the atomic index and move to backend.
v3: drop buffer decls. (Marek)
v4: fix off by one (Gert)
v5: fix off by one the other way (Dave)

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-By: Gert Wollny <gw.fossdev@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>