gpu/drm/exynos: Convert drm_atomic_helper_suspend/resume()

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

exynos_drm_fbdev_suspend/resume can be removed
as drm_mode_config_helper_suspend/resume has
implement the same in generic way.

Remove suspend_state from exynos_drm_private
struct as it is no more useful.

Signed-off-by: Ajit Negi <ajitn.linux@gmail.com>
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>