Move the UNIMPLEMENTED checks to where they are used with non-default
values. This allows dEQP to pass the prerequisite tests like state
reset without throwing assertion failures in debug.

BUG=angleproject:901,angleproject:512

Change-Id: I3bd35f6dea61e7d80d1379ba4e8e13437e68431a
Reviewed-on: https://chromium-review.googlesource.com/257130Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>