[graphite] Update UniformManager comment to address mat2x2 layout polyfill

Bug: b/318521741
Change-Id: I5fbdd496bcee753812a84e6ca7c6f2a707304285
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/801196
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Auto-Submit: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
1 file changed