zink: stop always syncing threaded flushes

async flushes are meant to be async here, and the other ones are manually
synced later in the function

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36643>

src/gallium/drivers/zink/zink_context.c

@@ -4078,7 +4078,7 @@ zink_flush(struct pipe_context *pctx,
       if (deferred && !(flags & PIPE_FLUSH_FENCE_FD) && pfence)
          deferred_fence = true;
       else
-         flush_batch(ctx, true);
+         flush_batch(ctx, false);
    }
 
    if (pfence) {

src/gallium/drivers/zink/zink_fence.c

@@ -129,11 +129,18 @@ tc_fence_finish(struct zink_context *ctx, struct zink_tc_fence *mfence, uint64_t
 static bool
 fence_wait(struct zink_screen *screen, struct zink_fence *fence, uint64_t timeout_ns)
 {
+   struct zink_batch_state *bs = zink_batch_state(fence);
    if (screen->device_lost)
       return true;
    if (p_atomic_read(&fence->completed))
       return true;
 
+   if (screen->threaded_submit) {
+      int64_t abs_timeout = os_time_get_absolute_timeout(timeout_ns);
+      if (!util_queue_fence_wait_timeout(&bs->flush_completed, abs_timeout))
+         return false;
+   }
+
    assert(fence->batch_id);
    assert(fence->submitted);
 
@@ -141,7 +148,7 @@ fence_wait(struct zink_screen *screen, struct zink_fence *fence, uint64_t timeou
 
    if (success) {
       p_atomic_set(&fence->completed, true);
-      zink_batch_state(fence)->usage.usage = 0;
+      bs->usage.usage = 0;
       zink_screen_update_last_finished(screen, fence->batch_id);
    }
    return success;