drm/msm: dsi host: Use device graph parsing to parse connected panel

drm/msm: dsi host: Use device graph parsing to parse connected panel

The dsi host looks for the connected panel node by parsing for a child
named 'panel'. This hierarchy isn't very flexible. The connected
panel is forced to be a child to the dsi host, and hence, a mipi dsi
device. This isn't suitable for dsi devices that don't use mipi dsi
as their control bus.

Follow the of_graph approach of creating ports and endpoints to
represent the connections between the dsi host and the panel connected
to it. In our case, the dsi host will only have one output port, linked
to the panel's input port.

Update DT binding documentation with device graph usage info.

v3:
- Fix return value checks of of_graph_* calls.
- Don't make port a mandatory DT property
- Fix defer check when no panel node specified
- Rename parse_dt func to align with other dsi_host funcs

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