[graphite] Add 565 format support

Bug: skia:12701
Change-Id: Ia51c9086c20d28af2fea254658730d9bd4179605
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/535396
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed