mesa/st: glsl_to_nir: don't lower atomics to SSBOs if driver supports HW atomics

mesa/st: glsl_to_nir: don't lower atomics to SSBOs if driver supports HW atomics

At least on r600 HW atomic operations are way less expensive than SSBO atomic
operations.

v2: use st->has_hw_atomics (Erik Anholt)

v3: remove second invocation of atomic to ssbo lowering (Erik Anholt)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3286>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3286>