Don't use mixed samples if NVPR is disabled

BUG=skia:

Change-Id: I48c72268e30e202a101177393eb777f18a11cb6f
Reviewed-on: https://skia-review.googlesource.com/8790
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
1 file changed