Makes the code more consistent and organized.

Also reduces binary size by about 4k.

Also refactors validation for GetBufferPointerv.

Bug: angleproject:2957
Change-Id: Ib076ce936193e9840eef2499bf815489cdb48479
Reviewed-on: https://chromium-review.googlesource.com/c/1350489
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>