usb: typec: altmodes/displayport: Make dp_altmode_notify() more generic

Make dp_altmode_notify() handle the dp->data.conf == 0 case too,
rather then having separate code-paths for this in various places
which call it.

Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Tested-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://lore.kernel.org/r/20210817215201.795062-8-hdegoede@redhat.com