Simplify prepare and set semantics (comments only)

Documentation updated for semantic changes vs. HWC 0.x:

* Prepare won't be called with NULL pointers. This used to be used to
  disable hardware composition, though that wasn't documented. Now
  we'll call prepare with non-NULL pointers but the layer list will
  have zero layers.

* Set won't be called with NULL pointers. This used to cause the
  display to turn off; that is now done by calling the blank() method,
  which is no longer optional.

Change-Id: I9c69dc34f64e499a5ba5f8729836e7c216f8c733
1 file changed