r600: fix refcnt imbalance related to atomic_buffer_state

r600: fix refcnt imbalance related to atomic_buffer_state

Indeed, these references are not freed.

For instance, this issue is triggered on an evergreen card with
"piglit/bin/shader_runner tests/spec/arb_shader_atomic_counter_ops/execution/all_touch_test.shader_test -auto -fbo"
while setting GALLIUM_REFCNT_LOG=refcnt.log.

Fixes: 06993e4ee350 ("r600: add support for hw atomic counters. (v3)")
Signed-off-by: Patrick Lerda <patrick9876@free.fr>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22798>