Vulkan prefers to use a single binding for each texture array.
Previously we were reserving binding space for array elements.

Doesn't immediately fix any tests but allows more tests to
pass in subsequent CLs.

Bug: angleproject:2494
Change-Id: I238d4e6f491bfa6daf04dde24f8111c8418cd59c
Reviewed-on: https://chromium-review.googlesource.com/1101570
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>