Set visible region dirty flag when alpha changes

Removes an optimzation which only set the
visible region dirty flag when alpha changed
from 1->0 or 0->1. This optimization broke an
assumption in CE. Alpha is only passed to HWC
when there are geometry changes which are
tracked by the visible region dirty flag.

Fixes: 299256533
Test: presubmit
Test: dim layer alpha animates when showing a dialog
Change-Id: I4c56171d19d582e26b9c2c7be6c555a6c1d7494c