Also use a single UpdateSubresource call to update the buffer.
Should improve performance on some benchmarks.

BUG=angleproject:1390

Change-Id: I70d54d86d3d3beb0e2caee86338ee03081070ac8
Reviewed-on: https://chromium-review.googlesource.com/663895Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>