drm/dp/mst: add some defines for logical/physical ports

This just removes the magic number.
Acked-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -1125,7 +1125,7 @@ static void drm_dp_add_port(struct drm_dp_mst_branch *mstb,
 			drm_dp_put_port(port);
 			goto out;
 		}
-		if (port->port_num >= 8) {
+		if (port->port_num >= DP_MST_LOGICAL_PORT_0) {
 			port->cached_edid = drm_get_edid(port->connector, &port->aux.ddc);
 			drm_mode_connector_set_tile_property(port->connector);
 		}

include/drm/drm_dp_helper.h

@@ -568,6 +568,10 @@
 #define MODE_I2C_READ	4
 #define MODE_I2C_STOP	8
 
+/* DP 1.2 MST PORTs - Section 2.5.1 v1.2a spec */
+#define DP_MST_PHYSICAL_PORT_0 0
+#define DP_MST_LOGICAL_PORT_0 8
+
 #define DP_LINK_STATUS_SIZE	   6
 bool drm_dp_channel_eq_ok(const u8 link_status[DP_LINK_STATUS_SIZE],
 			  int lane_count);