msm: vidc: wfd: Add WFD support for msm8974

Adds three new subdevices:
- Encoder subdevice to interface with new venus driver
    o Renamed enc-subdev.c to enc-mfc-subdev.c which only
      compiles for targets using the Samsung MFC core.
- MDP subdevice to interface with new MDSS driver
    o Renamed mdp-subdev.c to mdp-4-subdev.c used for targets
      using MDP4.
- MDP subdevice for debugging purposes
- Introduce a Kconfig file to properly choose between subdevices
  according to target

Change-Id: I125b39cfdeb1fbb4adf7c15d0c4452146764f985
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>