hwc: fixes for vsync and race conditions

- Fix some race conditions between hwc_eventControl and the hwcVsyncThread.
- don't call vsync-ctrl ioctl for same enable value
- move enable disable ioctls to vsync thread, this stabilizes the vsync
  signal
- read again if vsync read is interrupted

Change-Id: I4065918b352769dd9fe18bd0dff4a87053299189
CRs-Fixed: 393243
CRs-Fixed: 416487