Fix hash function for blendState cache

BUG=angleproject:1937

Change-Id: I0acd7dfcfb933b2b7ff0c1dc6854c7623ec57b3b
Reviewed-on: https://chromium-review.googlesource.com/453887Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>