drm: todo: remove task about switch to drm_connector_list_iter

This is now completed.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170511191049.28944-9-gustavo@padovan.org