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>
Link: https://patchwork.freedesktop.org/patch/msgid/ <3c8b030bb89ec5aeafdb3c294cb6b3403d8c0601.1566845537.git.andrzej.p@collabora.com