drm/msm/dsi: Add a mdp4 encoder for DSI

Create an mdp4 incoder for DSI. Only DSI video mode is supported as of
now.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Vinay Simha BN <vinaysimha@inforcecomputing.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>