Process all display events in order

Display events in each batch received from IPC were being processed in
reverse order, and stopped after the first vsync event (latest
chronologically) was handled. This makes perfect sense for vsync
events, but is broken for hotplug events.

Now we process them all in order, handling all except vsync as we see
them. For vsync events, only the last is reported.

Bug: 7491120
Change-Id: I448d139d21dc27128d75ca1d661de666fff51bcb