hwc: Query MDP capability for blending stages supported.

- Read MDP sys_fs node for blending stages supported per mixer
  instead of hard cording the value. This makes the implementation
  scalable across chipsets.
- Add binder support to change the max pipes per layer value
  dynamically for MDP comp.

Change-Id: I3b83f4bdc2e2e81ca55c54fc6bcfd61c55fef501
8 files changed