[graphite] Assign kVertexShader a real value

* Small adjsutment which should fix validation layer errors introduced from https://skia-review.googlesource.com/c/skia/+/789216/11

Change-Id: I9af90f2ce32063eb40efd77948d1c8018f42542b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/791760
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Auto-Submit: Nicolette Prevost <nicolettep@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
1 file changed