drm: rockchip: set alpha_en to 0 if it is not used

alpha_en should be set to 0 if it is not used, i.e. to disable alpha
blending if it was enabled before and should be disabled now.

Fixes: 2aae8ed1 ("drm/rockchip: Add per-pixel alpha support for the PX30 VOP")
Signed-off-by: Alex Bee <knaerzche@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210528130554.72191-6-knaerzche@gmail.com

drivers/gpu/drm/rockchip/rockchip_drm_vop.c

@@ -1022,6 +1022,7 @@ static void vop_plane_atomic_update(struct drm_plane *plane,
 		VOP_WIN_SET(vop, win, alpha_en, 1);
 	} else {
 		VOP_WIN_SET(vop, win, src_alpha_ctl, SRC_ALPHA_EN(0));
+		VOP_WIN_SET(vop, win, alpha_en, 0);
 	}
 
 	VOP_WIN_SET(vop, win, enable, 1);