drm/msm/dsi: Modify port parsing

The DSI interface is going to have two ports defined in its device node.
The first port is always going to be the link between the MDP output
and the input to DSI, the second port is going to be the link between
the DSI output and the connected panel/bridge:

 -----           -----           -------
| MDP | ------> | DSI | ------> | Panel |
 -----           -----           -------
        (Port 0)       (Port 1)

Until now, there was only one Port representing the output. Update the
DSI host driver such that it parses Port #1 for a connected device.

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