media/gpu/V4L2StatefulVD: Decrement |num_decoder_instances_| in Reset()

https://crrev.com/c/5398807 modified the Reset() routine to basically
destroy both OUTPUT and CAPTURE queues, and allowed a hypothetical
subsequent Decode() to call Initialize() if needed. Initialize() has a
global |num_decoder_instances_|; said CL forgot to decrement it in
the Reset(). This CL does that, fixing the Tast video.Seek.* on trogdor.

Bug: 328002324
Change-Id: If0c81246d5a0accdf3d50033f190edcc248f5437
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5423008


Auto-Submit: Miguel Casas-Sanchez <mcasas@chromium.org>
Commit-Queue: Miguel Casas-Sanchez <mcasas@chromium.org>
Commit-Queue: Nathan Hebert <nhebert@chromium.org>
Reviewed-by: Nathan Hebert <nhebert@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282329}