Disable --gpuThreads when skpbenching DDLs

This CL requires https://skia-review.googlesource.com/c/skia/+/136175 (Add --gpuThreads support to skpbench.py (take 2))

Change-Id: Iced00d6b6184ec41efe66a71a5b4e062ce7205ee
Reviewed-on: https://skia-review.googlesource.com/136596
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
3 files changed