drm: Update todo.rst

Internship season is starting, let's review this. One thing that's
pending is Maxime's work to roll out drm_atomic_state pointers to all
callbacks, he said he'll remove that entry once it's all done.

v2: Fix typos (Maxime)

Acked-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210121112919.1460322-1-daniel.vetter@ffwll.ch