asahi: Fix batch writer_syncobj cleanup

When an ACTIVE batch takes over the active writer role from a SUBMITTED
batch, the written BO has the syncobj from the latter even though the
writer is the former. This is correct and an intended state, but it
means that then we can't gate the syncobj cleanup in agx_batch_cleanup
on being the active writer, since the SUBMITTED batch won't be.

Fixes: asahi/mesa#18

Signed-off-by: Asahi Lina <lina@asahilina.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22891>

src/gallium/drivers/asahi/agx_batch.c

@@ -210,10 +210,12 @@ agx_batch_cleanup(struct agx_context *ctx, struct agx_batch *batch, bool reset)
 
          if (writer == batch) {
             assert(bo->writer_syncobj == batch->syncobj);
-            bo->writer_syncobj = 0;
             agx_writer_remove(ctx, handle);
          }
 
+         if (bo->writer_syncobj == batch->syncobj)
+            bo->writer_syncobj = 0;
+
          agx_bo_unreference(agx_lookup_bo(dev, handle));
       }
    }