hwc/overlay: Add support for pipe priorities, 1 pipe config for FB

If using source split to stage 2 pipes on the same mixer stage,
the left pipe needs to be a higher priority than the right.

Add API in overlay to compare pipe priorities and use this in
source split config to stage pipes accordingly.

Add support for 1 pipe config for FB if updating rect is within 2048
pixels.

Change-Id: I8b33d5ebd0f8765ee842bab128d8abd67a110145
9 files changed