drm/rockchip: vop: fix window origin calculation

VOP_WINx_DSP_ST does not require subtracting 1 from the values written to
it. It actually causes the screen to be shifted by one pixel.

Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Tested-by: Yakir Yang <ykk@rock-chips.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Dominik Behr <dbehr@chromium.org>
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>