Break apart public and private gpu files in GN lists

In order to make Chrome's build of Skia gn check compliant,
we need to break apart a few of our gni file lists so some
files can be listed as "public" headers, instead of all
being grouped together in "sources".

https://chromium-review.googlesource.com/c/chromium/src/+/4089412
sketched out what changes were needed and this implements
those changes.

I'm not sure why the gpu.gni file re-alphabetized a bunch
of files (AFAIK, the gn binary version was not changed), but
as long as gn format is happy, I'm happy.

Bug: skia:12541, skia:13983
Change-Id: I95409479d8f066f7069f6aa5e1112dede233dd4f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/627988
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
11 files changed