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

drivers/gpu/drm/display/drm_dp_mst_topology.c

@@ -2274,7 +2274,7 @@ drm_dp_mst_port_add_connector(struct drm_dp_mst_branch *mstb,
 
 	if (port->pdt != DP_PEER_DEVICE_NONE &&
 	    drm_dp_mst_is_end_device(port->pdt, port->mcs) &&
-	    port->port_num >= DP_MST_LOGICAL_PORT_0)
+	    drm_dp_mst_port_is_logical(port))
 		port->cached_edid = drm_edid_read_ddc(port->connector,
 						      &port->aux.ddc);
 
@@ -4219,7 +4219,7 @@ drm_dp_mst_detect_port(struct drm_connector *connector,
 	case DP_PEER_DEVICE_SST_SINK:
 		ret = connector_status_connected;
 		/* for logical ports - cache the EDID */
-		if (port->port_num >= DP_MST_LOGICAL_PORT_0 && !port->cached_edid)
+		if (drm_dp_mst_port_is_logical(port) && !port->cached_edid)
 			port->cached_edid = drm_edid_read_ddc(connector, &port->aux.ddc);
 		break;
 	case DP_PEER_DEVICE_DP_LEGACY_CONV:
@@ -5983,7 +5983,7 @@ static bool drm_dp_mst_is_virtual_dpcd(struct drm_dp_mst_port *port)
 		return false;
 
 	/* Virtual DP Sink (Internal Display Panel) */
-	if (port->port_num >= 8)
+	if (drm_dp_mst_port_is_logical(port))
 		return true;
 
 	/* DP-to-HDMI Protocol Converter */

include/drm/display/drm_dp_mst_helper.h

@@ -949,6 +949,12 @@ int __must_check drm_dp_mst_root_conn_atomic_check(struct drm_connector_state *n
 void drm_dp_mst_get_port_malloc(struct drm_dp_mst_port *port);
 void drm_dp_mst_put_port_malloc(struct drm_dp_mst_port *port);
 
+static inline
+bool drm_dp_mst_port_is_logical(struct drm_dp_mst_port *port)
+{
+	return port->port_num >= DP_MST_LOGICAL_PORT_0;
+}
+
 struct drm_dp_aux *drm_dp_mst_dsc_aux_for_port(struct drm_dp_mst_port *port);
 
 static inline struct drm_dp_mst_topology_state *