dt-bindings: display: Use OF graph schema

Now that we have a graph schema, rework the display related schemas to use
it. Mostly this is adding a reference to graph.yaml and dropping duplicate
parts from schemas.

In panel-common.yaml, 'ports' is dropped. Any binding using 'ports'
should be one with more than 1 port node, and the binding must define
what each port is.

Note that ti,sn65dsi86.yaml, ti,tfp410,yaml and toshiba,tc358768.yaml will
need further updates to use video-interfaces.yaml once that lands.

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210104180724.2275098-1-robh@kernel.org