overlay: Add support for decimation.

Add support for decimation on top of what MDP can downscale,
because decimation can have quality issues.
B-family MDP downscale of 1/4, plus decimation of 1/16 can let
us have a total downscale of 1/64.

Also decimate by 2 (min) if width is greater than what layer mixer
supports and is not handled by the client.

Change-Id: I3b7c4bf321c7561bd98206ad118f4ac1ee2879ae
7 files changed