drm/rockchip: vop2: Fix cluster windows alpha ctrl regsiters offset

The phy_id of cluster windws are not increase one for each window.

Fixes: 604be855 ("drm/rockchip: Add VOP2 driver")
Tested-by: Derek Foreman <derek.foreman@collabora.com>
Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20241209122943.2781431-6-andyshrk@163.com