r600: amend space check for chips older than EVERGREEN

r600: amend space check for chips older than EVERGREEN

evergreen_emit_atomic_buffer_setup_count is only called if chip >= EVERGREEN
otherwise atomic_used_mask is left uninitialized when unconditionally used by
r600_need_cs_space so it might want more space than needed

fix this by always initializing atomic_used_mask

Fixes: 32529e60849 ("r600/eg: rework atomic counter emission with flushes")
Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7417>