drm/rockchip: Use for_each_oldnew_plane_in_state in vop_crtc_atomic_flush

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

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Mark Yao <mark.yao@rock-chips.com>
Cc: Heiko Stuebner <heiko@sntech.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-rockchip@lists.infradead.org
Link: http://patchwork.freedesktop.org/patch/msgid/20170712081344.25495-10-maarten.lankhorst@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Mark Yao <mark.yao@rock-chips.com>