[graphite] Move MSL creation into the ProgramInfo

Bug: skia:12466
Change-Id: If31d25a1f4a4805069f6556c6802f33fa0bdfcc1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/464380
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
3 files changed