drm/dp_mst: fix drm_dp_mst_helper.h kernel-doc

Drop excess vcpi member documentation.
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/606eee280718ba372093fdebbda42c7581dbd827.1709749576.git.jani.nikula@intel.com

include/drm/display/drm_dp_mst_helper.h

@@ -83,7 +83,6 @@ struct drm_dp_mst_branch;
  * @passthrough_aux: parent aux to which DSC pass-through requests should be
  * sent, only set if DSC pass-through is possible.
  * @parent: branch device parent of this port
- * @vcpi: Virtual Channel Payload info for this port.
  * @connector: DRM connector this port is connected to. Protected by
  * &drm_dp_mst_topology_mgr.base.lock.
  * @mgr: topology manager this port lives under.