drm/todo: Plumb drm_atomic_state all over

Add a TODO for plumbing drm_atomic_state all over to ease
the hurdles of accessing additional object states.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> #irc
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200717135439.5996-1-ville.syrjala@linux.intel.com