drm/dp_mst: Use DP_MST_LOGICAL_PORT_0 instead of magic number

Use the macro defined for the first logical port instead of the
corresponding magic number.

Cc: Lyude Paul <lyude@redhat.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210201120145.350258-4-imre.deak@intel.com

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -4219,9 +4219,8 @@ 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 >= 8 && !port->cached_edid) {
+		if (port->port_num >= DP_MST_LOGICAL_PORT_0 && !port->cached_edid)
 			port->cached_edid = drm_get_edid(connector, &port->aux.ddc);
-		}
 		break;
 	case DP_PEER_DEVICE_DP_LEGACY_CONV:
 		if (port->ldps)