drm/radeon: 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: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0836f34238730afce3f4d6b13f5cf04f832b668a.1564161140.git.andrzej.p@collabora.com