[graphite] Minor DrawAtlas cleanup.

Mainly removing some duplicated code and placing related things together

Change-Id: I82c514a5c2430ac37bc80a6b730f869042bdb3a7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/819697
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed