drm/msm/mdp5: Move *_modeset_init out of construct_encoder function

This change is to make the content in construct_encoder reflect its
name.
Also, DSI connector may be connected to video mode or command mode
encoder, so that 2 different encoders need to be constructed for DSI.

Signed-off-by: Hai Li <hali@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>