streamline interface to GrColorSpaceXform::Make()

We can now pass all cs/at params, so no need for UnpremulToUnpremul().

Also found a spot to use sk_srgb_singleton().

Does this make sense to apply to GrColorSpaceXformEffect too?

Change-Id: I1ee1f5919aa2ae841a98eeb5662cb80c244b1049
Reviewed-on: https://skia-review.googlesource.com/148665
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
9 files changed