drm/msm/mdp4: Initialize DSI encoders

Create DSI encoders during modeset_init. The 2 encoders should ideally be
one command mode and one video mode DSI encoder respectively, but we don't
support command mode yet. We just create 2 of the same because the dsi
driver expects it, we end up using only the first one.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>