drm: TODO: Add DRM_MODESET_LOCK_ALL* conversion to todo.rst

We should also get the drivers using the helpers. Seems like a good
intermediate TODO item.

Changes in v2:
- Add line about replacing drm_modest_lock_all() (Daniel)

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181129195838.222031-1-sean@poorly.run