Avoid unnecessary calls to updateTransformHint

It was observed that this could start to take
noticeable time with high layer counts, I guess
due to the acquisition of one mutex per layer per call. Anyway
it looks like there's no need to call it if a display transaction
didn't occur.

Test: Existing tests pass.
Change-Id: I1918d326a6d0ed32a0a5fea4008a746d328b335d