hwc: Add support for SourceSplit

Add support for SourceSplit. The mixers are abstracted from hwc.
There would be no translations to mixer understood destinations.

1) If a layer's crop and position are < 2048, only 1 pipe can be used
irrespective of position.
2) Else 2 pipes can be used with the layer equally split, without
regard to position.

Change-Id: I501a53838d147887c1e2299366663e05320d5096
6 files changed