hwc: Handle more events from driver

In certain cases, such as the shutdown sequence, the driver may
blank itself without any direction from SurfaceFlinger.
In such cases the HWC should make sure that no further updates on
the display occur.
Handle events from driver in a more generic way to include blank
events and leave room for any other events that need to be polled
on from the driver.

Change-Id: Iabc91a70e71b42d3a7319504ae70b96908acbffe
CRs-fixed: 627814
1 file changed