This reduces the code duplication in this json file. It should
make the packed enum map easier to maintain going forward as we
add more entry point permutations.

Bug: angleproject:3611
Change-Id: Ideab630584f0a41892262660fc1e0aedcc5dd639
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1736126Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>