drm/todo: Add levels

Should help new people pick suitable tasks.

Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Sean Paul <sean@poorly.run>
Reviewed-by: Sean Paul <sean@poorly.run>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022152530.22038-2-daniel.vetter@ffwll.ch