[graphite] Simplify how blend modes are stored in the CombinationBuilder

Bug: skia:12701
Change-Id: I88a12b776cf9b14d0c0a94ae777b85d9592696a7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/544318
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed