Superres should not be used together with intrabc, for the same
reasons why the other loop filters are disabled when intrabc is
used. However, we actually read superres information first,
so the simplest change is to only read allow_intrabc if superres is off.

BUG=aomedia:1384

Change-Id: I55c4c8ace72ab3fac9417da5cf803883be7efb84