drm/msm: Add display components by parsing MDP ports

The kms driver currently identifies all the mdss components it needs by
parsing a phandle list from the 'connectors' DT property.

Instead of this, describe a list of ports that the MDP hardware provides
to the external world. These ports are linked to external encoder
interfaces such as DSI, HDMI. These are also the subcomponent devices
that we need add. This description of ports complies with the generic
graph bindings.

The LVDS port is a special case since it is a part of MDP4 itself, and
its output connects directly to the LVDS panel. In this case, we don't
try to add it as a component.

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