fixed incorrect index in SPIR-V matrix construction

Bug: skia:7908
Change-Id: I990e5f6e84afdc81d340f433d93a9061b9dd67ef
Reviewed-on: https://skia-review.googlesource.com/127397
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
1 file changed