hwcomposer : Synchronize EventThread, and hwcVsyncThread

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 keeps
using CPU.

Change-Id: Ic59d0c477bc8483dd731777ebde3cc9e12bb681a
CRs-Fixed: 393243