drm: kernel-doc: document drm_dp_set_subconnector_property() params

Changeset e5b92773 ("drm: report dp downstream port type as a subconnector property")
added a new function to the kAPI, but didn't add any documentation
for the parameters for drm_dp_set_subconnector_property().

Fixes: e5b92773 ("drm: report dp downstream port type as a subconnector property")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/0870be85a77bea4ba5cf1715010834289a4e10b1.1603791716.git.mchehab+huawei@kernel.org