dt-bindings: display: mediatek, dpi: Convert to use graph schema

Update the mediatek,dpi binding to use the graph schema. Missed
this one from the mass conversion since it's not part of drm-misc.

Cc: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: CK Hu <ck.hu@mediatek.com>
Cc: Jitao shi <jitao.shi@mediatek.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

Documentation/devicetree/bindings/display/mediatek/mediatek,dpi.yaml

@@ -50,15 +50,10 @@ properties:
       - const: sleep
 
   port:
-    type: object
+    $ref: /schemas/graph.yaml#/properties/port
     description:
-      Output port node with endpoint definitions as described in
+      Output port node. This port should be connected to the input port of an
-      Documentation/devicetree/bindings/graph.txt. This port should be connected
+      attached HDMI or LVDS encoder chip.
-      to the input port of an attached HDMI or LVDS encoder chip.
-
-    properties:
-      endpoint:
-        type: object
 
 required:
   - compatible