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