Synchronization is needed,to make sure, read() is not called
by hwcVsyncThread when VSYNC is disabled.
Otherwise in usecases, where display is OFF, and still some
tasks keep running in the background; SurfaceFlinger hogs
CPU.
(cherry picked from commit ffa84468bbd2517ffb22ab4023cb57a6272fc80d)
Change-Id: Idd779647a27ed3e6fb0703a07033dd5f7cd1b5fd