By using the helper isTransformFeedbackActive* functions.

Bug: angleproject:3205
Change-Id: I50b3e9db427da9f65c3e2b50d7847464e8c28202
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1665353
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>