drm/dp: add drm_dp_link_power_down() helper

We had _power_up(), but drivers also need to be able to power down.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index 11f8c84..7e25030 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -586,6 +586,7 @@
 
 int drm_dp_link_probe(struct drm_dp_aux *aux, struct drm_dp_link *link);
 int drm_dp_link_power_up(struct drm_dp_aux *aux, struct drm_dp_link *link);
+int drm_dp_link_power_down(struct drm_dp_aux *aux, struct drm_dp_link *link);
 int drm_dp_link_configure(struct drm_dp_aux *aux, struct drm_dp_link *link);
 
 int drm_dp_aux_register(struct drm_dp_aux *aux);