[dm] Fix TSAN reports about "done"

Change-Id: Id153febe3bc2e4da0f487112c9f65ad2d715dc90
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/836018
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: Ben Wagner <bungeman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>