drm/dp: fix kernel-doc warnings at drm_dp_helper.c

As warned by kernel-doc:

	./drivers/gpu/drm/drm_dp_helper.c:385: warning: Function parameter or member 'type' not described in 'drm_dp_downstream_is_type'
	./drivers/gpu/drm/drm_dp_helper.c:886: warning: Function parameter or member 'dev' not described in 'drm_dp_downstream_mode'

Some function parameters weren't documented.

Fixes: 38784f6f ("drm/dp: Add helpers to identify downstream facing port types")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200922175357.42998-2-lyude@redhat.com