gallium/trace: Fix helgrind complaint about one-time init

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7644>

src/gallium/auxiliary/driver_trace/tr_screen.c

@@ -670,15 +670,11 @@ trace_screen_destroy(struct pipe_screen *_screen)
 bool
 trace_enabled(void)
 {
-   static bool firstrun = true;
-
-   if (!firstrun)
-      return trace;
-   firstrun = false;
-
-   if(trace_dump_trace_begin()) {
-      trace_dumping_start();
-      trace = true;
+   do_once {
+      if(trace_dump_trace_begin()) {
+         trace_dumping_start();
+         trace = true;
+      }
    }
 
    return trace;