drm/dp_mst: Fill branch->num_ports

This field on drm_dp_mst_branch was never filled

It is initialized to zero when the port is kzallocced.
When a port is added to the list, increment num_ports,
and when a port is removed from the list, decrement num_ports.

v2: remember to decrement on port removal
v3: don't explicitly init to 0
v4: move decrement of num_ports to unlink_port function

Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: David Francis <David.Francis@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -2175,6 +2175,7 @@ drm_dp_mst_topology_unlink_port(struct drm_dp_mst_topology_mgr *mgr,
 				struct drm_dp_mst_port *port)
 {
 	mutex_lock(&mgr->lock);
+	port->parent->num_ports--;
 	list_del(&port->next);
 	mutex_unlock(&mgr->lock);
 	drm_dp_mst_topology_put_port(port);
@@ -2274,6 +2275,7 @@ drm_dp_mst_handle_link_address_port(struct drm_dp_mst_branch *mstb,
 		mutex_lock(&mgr->lock);
 		drm_dp_mst_topology_get_port(port);
 		list_add(&port->next, &mstb->ports);
+		mstb->num_ports++;
 		mutex_unlock(&mgr->lock);
 	}