drm/dp_mst: Factor out drm_dp_mst_port_is_logical()

Factor out a function to check if an MST port is logical, used by a
follow-up i915 patch in the patchset.

v2: Move drm_dp_mst_aux_for_parent() forward declaration to the next
    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/20240416221010.376865-9-imre.deak@intel.com