drm/amdgpu: Provide ddc symlink in dm connector's sysfs directory

Use the ddc pointer provided by the generic connector.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ <40293fa4e619d1d1af213a076b1d03440e50c56c.1566845537.git.andrzej.p@collabora.com