drm/dp: Add missing kernel-doc

Commit 9dc40560 (drm/dp: let drivers specify the name of the I2C-
over-AUX adapter) introduced a new field but didn't add the proper
kernel-doc for it.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>