drm/connector: Fix typo in drm_connector_list_iter_next()

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180716171711.413-1-lyude@redhat.com