DispSync: remove delay when enabling vsync events

This change fixes a bug that caused an extra frame of latency when enabling
vsync event callbacks in DispSync.  The bug was related to the logic that
prevents the two events from firing with very little time between them due to
updates to the vsync model.

Bug: 11479720
Change-Id: Ie7eaff9e92ffb7b7b6cb4d3d4402c96cbd29af7e