This removes another instance of indexing a flat array by VkFormat.
With the introduction of YUV formats we no longer have a compact
table. Switching to ANGLE format indexing allows us to keep a flat
array and avoid using an unordered map.

Bug: angleproject:5438
Change-Id: I96caa19e3b7ce419ce09680399919447f002b0bd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2622238Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>