DispSync: Always resync after inactivity

Changes DispSync to enable hardware vsync immediately when new frames
arrive after a period of inactivity.

No matter how hard we try, we can't avoid drifting over time without
being able to detect error based on display retire fences. By enabling
hardware vsync immediately, we avoid having a weird period or phase
offset relative to hardware while we retrain the model. Once the model
has locked, we turn hardware vsync back off to save power (until we
detect drift again).

Bug: 26255070
Change-Id: If4dd17c2d541015c730f47d824359d7cb4b52c3c