panfrost: signal syncobj if nothing is going to be flushed

When nothing is going to be flushed, the kernel will get no job that
signals the syncobj.

Signal it by ourselves, otherwise it will never get signaled.

Closes: #3371

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6190>

src/gallium/drivers/panfrost/pan_job.c

@@ -1076,8 +1076,11 @@ panfrost_batch_submit(struct panfrost_batch *batch, uint32_t out_sync)
         int ret;
 
         /* Nothing to do! */
-        if (!batch->scoreboard.first_job && !batch->clear)
+        if (!batch->scoreboard.first_job && !batch->clear) {
+                if (out_sync)
+                        drmSyncobjSignal(dev->fd, &out_sync, 1);
                 goto out;
+	}
 
         panfrost_batch_draw_wallpaper(batch);