drm/rockchip: Convert to using __drm_atomic_helper_crtc_reset() for reset.

Convert rockchip to using __drm_atomic_helper_crtc_reset(), instead of
writing its own version. Instead of open coding
destroy_state(), call it directly for freeing the old state.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Sandy Huang <hjc@rock-chips.com>
Cc: "Heiko Stübner" <heiko@sntech.de>
Cc: linux-rockchip@lists.infradead.org
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20190301125627.7285-14-maarten.lankhorst@linux.intel.com