drm/rockchip: vop2: add support to synchronize VPs automatically

For some application scenario, such as multi-screen splicing, multiple
VPs should display synchronously.

To Synchronize VP0 and VP1, the DTS configurations may be like:

&vop {
	rockchip,sync-vp-mask = /bits/ 8 <3>;
};

The 8-bits value 3 means VP0(1 << 0) and VP1(1 << 1).

Signed-off-by: Damon Ding <damon.ding@rock-chips.com>
[rebase]
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>