drm/mediatek: Provide ddc symlink in hdmi connector sysfs directory

Use the ddc pointer provided by the generic connector.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/ <db323a3cdfd643cef65d796e959c16a14f105920.1564591626.git.andrzej.p@collabora.com