gpu: fix parsing of third descriptor in chain
gpu: fix parsing of third descriptor in chain

Descriptor chains of three descriptors were not having the last
descriptor parsed correctly which resulted in some commands not
properly being responded to.

BUG=None
TEST=build with --features=gpu; null_platform_test

Change-Id: Ia77c7ca485ded7cdcb71fd62e8f6be5c226df1bf
Reviewed-on: https://chromium-review.googlesource.com/1073956
Commit-Ready: David Riley <a1c52a99f156ff41f0ea478f29291144551a02f6@chromium.org>
Tested-by: David Riley <a1c52a99f156ff41f0ea478f29291144551a02f6@chromium.org>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm