CachedSet::hasBufferUpdate: remove iterator param

Bug: 183735906
Test: libcompositionengine_test

Although this iterator was incremented, its value is never accessed, and
it is just a copy of the caller's iterator. Removing it has no effect
besides simplifying the code.

Change-Id: Iab37107d199c8b441184a49d35a34bc67293b741