gallium/tgsi: start adding hw atomics (v3.2)

gallium/tgsi: start adding hw atomics (v3.2)

This adds support for a hw atomic counters to TGSI.

A new register file for storing atomic counters is added,
along with a new atomic counter semantic, along with docs
for both.

v2: drop semantic, move hw counter to backend,
Ilia pointed out SSO would have busted my plan, and he
was right.
v3: drop BUFFER decls. (Marek)
v3.1: minor fixups for whitespace, set ureg error
if we overflow the hw atomic limits. (nha)
v3.2: fix some docs inconsistencies (Ilia)

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