drm/rockchip: vop2: Fix the mixer alpha setup for layer 0

The alpha setup should start from the second layer, the current calculation
starts incorrectly from the first layer, a negative offset will be obtained
in the following formula:

offset = (mixer_id + zpos - 1) * 0x10

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/20241209122943.2781431-7-andyshrk@163.com