Unlock previously locked overlay buferrs in N+1th draw round, rather than N+2th.
This is now possible because of wait for PAN.
Provides an opportunity to improve genlock behavior and makes it more ICS like.
(cherry picked from commit b3a4cd7944397565388b4e940fb41d2af8882687)
Change-Id: I48f937b77d59b31f3bf04c5a2eb35500bd5d76d6