drm/rockchip: update cursors asynchronously through atomic.

Add support to async updates of cursors by using the new atomic
interface for that.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
[updated for upstream]
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20181205123310.7965-1-helen.koike@collabora.com