drm/bridge: dumb-vga-dac: Provide ddc symlink in 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: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ebb75e71b8b7c8d65d54a947a03fd21b8969fb3a.1564161140.git.andrzej.p@collabora.com