drm/mediatek: Convert to new iterator macros

drm/mediatek: Convert to new iterator macros

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: CK Hu <ck.hu@mediatek.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Link: http://patchwork.freedesktop.org/patch/msgid/20170712081344.25495-14-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>