drm/tilcdc: Convert drm_atomic_helper_suspend/resume()

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

saved_state in tilcdc_drm_private will not be used
anymore, so it can be removed.

Signed-off-by: Ajit Negi <ajitn.linux@gmail.com>
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Tested-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>