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: 06993e4e ("r600: add support for hw atomic counters. (v3)")

Signed-off-by: Patrick Lerda patrick9876@free.fr