drm/dp_mst: Add drm_dp_mst_aux_for_parent()

Add a function to get the AUX device of the parent of an MST port, used
by a follow-up i915 patch in the patchset.

v2: Move drm_dp_mst_aux_for_parent() forward declaration to this patch
    (Ankit)

Cc: Lyude Paul <lyude@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240416221010.376865-10-imre.deak@intel.com