drm/imx: Use atomic 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: Philipp Zabel <p.zabel@pengutronix.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20170712081344.25495-13-maarten.lankhorst@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de>