drm/imx: Convert drm_atomic_helper_suspend/resume()

convert drm_atomic_helper_suspend/resume() to use
drm_mode_config_helper_suspend/resume().

with this conversion, the remaining member of struct
imx_drm_device, state, will be no more useful and it
could be removed forever.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Ajit Negi <ajitn.linux@gmail.com>
[p.zabel@pengutronix.de: rebased onto drm-next, updated commit message]
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>