drm/doc: Add todo about connector_list_iter

At least radeon, amdgpu and nouveau should be converted. We have
patches for i915 already.

v2: Spelling (Sean).

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170308141257.12119-2-daniel.vetter@ffwll.ch