Revert "[graphite] Remove unnecessary premul step in YUV shaders."

This reverts commit 0c778603715e6062d531bde211c4ea74e37e12df.

Reason for revert: Running MotionMarkBouncingTaggedImages with
this is 4ms slower (25%) on my M1. I'm not sure why, so reverting
for now.

Original change's description:
> [graphite] Remove unnecessary premul step in YUV shaders.
>
> If we do a colorspace transformation we end up unpremul-ing the result
> anyway, and raw YUV image shaders should be relatively rare.
>
> Bug: b/352537244
> Change-Id: I4b5f0636c9ff6b42376db8246993b1853960bfd6
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/876458
> Reviewed-by: Robert Phillips <robertphillips@google.com>
> Commit-Queue: Jim Van Verth <jvanverth@google.com>
> Reviewed-by: Michael Ludwig <michaelludwig@google.com>

Bug: b/352537244
Change-Id: I902f3f1a6a51be753bcaf96ca41f0048900d64dd
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/877418
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>