In the process, cleanup some of the maths and functionality used to setup
descriptors and compute parameters. Also cleanup and correct the context
passing.

Change-Id: I6b6629bc81ef1c03425332dd6eadf3085efec7c9
Reviewed-on: https://chromium-review.googlesource.com/588892Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Daniel Clifford <danno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46938}