Reduces the buffer nullptr checks to a single location. Also optimizes
how the transform feedback binding counter is changed.

Improves the score of the draw call vertex array change test.

Bug: angleproject:3014
Change-Id: I08ff341e08194a407c24143965a3d568e92b97b7
Reviewed-on: https://chromium-review.googlesource.com/c/1406891
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>