drm: rcar-du: Use new iterator macros

for_each_obj_in_state is about to be removed, so use the correct new
iterator macros.

Also look at new_plane_state instead of plane->state when looking up
the hw planes in use. They should be the same except when reallocating,
(in which case this code is skipped) and we should really stop looking
at obj->state whenever possible.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>