drm/msm/mdp4: Clean up modeset_init

modeset_init() for mdp4 isn't very flexible. That makes it hard to add
more interfaces.

Split out the encoder/connector creation code in modeset_init into a
separate function. This is similar to what's done in modeset_init for
mdp5.

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