2D array texture's robust resource init didn't work due to
mtl::InitializeTextureContents() expects one slice at a time, however
TextureMtl::initializeContents() created entire layers index via
GetZeroLevelIndex() before passing to mtl::InitializeTextureContents().
Fixed by changing to GetLayerMipIndex() instead.

Bug: angleproject:2634
Change-Id: Ie8d80d927a987c2b393d913fda8288b6e06e97d0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2494527Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com>