drm/rockchip: Convert drm_atomic_helper_suspend/resume()

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

With this conversion, rockchip_drm_fb_resume() and
rockchip_drm_fb_suspend() will not be used anymore.
Both of these functions can be removed.

Also, in struct rockchip_drm_private state will not be
used anymore. So this can be removed forever.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Co-Developed-by: Ajit Negi <ajitn.linux@gmail.com>
[changed to Co-Developed-by, according to process/submitting-patches.rst]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180731203430.GA30136@jordon-HP-15-Notebook-PC