hwc: Implement idle-fallback for videos

In case of videos, do a fallback using 2 pipes per mixer at most.
This redraw will happen only once to reduce pipes from at most 4
to at most 2 and never draw again until there is an update.

Change-Id: I78c0dc2af4e3bd7e9128994e507e777187b4e247
2 files changed