[graphite] Add CommandBuffer creation

Bug: skia:12466
Change-Id: I3a22d03e5042a94f2bf6d379b2fff17f0b48edb3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/455217
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
8 files changed