dt-bindings: display: Fix graph 'unevaluatedProperties' related warnings

The graph schema doesn't allow custom properties on endpoint nodes for
'#/properties/port' and '#/$defs/port-base' should be used instead. This
doesn't matter until 'unevaluatedProperties' support is implemented.

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Marek Vasut <marex@denx.de>
Cc: Krishna Manikandan <mkrishn@codeaurora.org>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://lore.kernel.org/r/20210719195001.2412345-1-robh@kernel.org