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: I0aab9f30046f07ca9c90c73cadabcadc150565c8
CRs-fixed: 627814