drm: drop drm_bus from todo

David Herrmann removed the last bits of drm_bus in:
commit c5786fe5 ("drm: Goody bye, drm_bus!")

Remove the todo item.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com> [I miss drm_bus!]
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20190126192732.15263-1-sam@ravnborg.org
Link: https://patchwork.freedesktop.org/patch/msgid/20190126192732.15263-1-sam@ravnborg.org