[graphite] Better encapsulate SkPipelineData's uniform data

We are going to cache the uniform data and the texture data separately.
Begin by making the uniform data something that could be cached on its own.

Bug: skia:12701
Change-Id: If3ea3a9b6050faf0810549d4076ae44732656a9e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/522178
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
4 files changed