DispSync: Don't resync if HW vsync is unavailable

Don't attempt to resync to hardware vsync if it's not available (if
the display is blanked), which should prevent unusual sequences of
commands (enable vsync, then unblank) to hardware composer.

Bug: 28177844
Change-Id: I9d6046fb0a346830e98829fd250642f0809d7e31
2 files changed