drm/todo: Add entry to rename drm_atomic_state

The name of the structure drm_atomic_state is confusing. Let's add an
entry to our todo list to rename it.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20231214100917.277842-5-mripard@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>