drm/rockchip: Use atomic PM helpers

This driver was still using the old legacy helpers and that caused a few
NULL dereferences when trying to call empty callbacks.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Caesar Wang <wxt@rock-chips.com>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Heiko Stuebner <heiko@sntech.de>
Cc: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465224813-7359-1-git-send-email-tomeu.vizoso@collabora.com