drm/rockchip: check yuv2yuv existence before assigning window data

Before assigning window data, we should check if the yuv2yuv vop-data
is set at all, because it looks like it can otherwise reference something
wrong, as I saw on my rk3188 today which ended up in a null pointer
dereference in vop_plane_atomic_update when accessing the yuv2yuv data.

Fixes: 1c21aa8f ("drm/rockchip: Fix YUV buffers color rendering")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2556882.Heuq80WCVD@phil

drivers/gpu/drm/rockchip/rockchip_drm_vop.c

@@ -1675,7 +1675,9 @@ static void vop_win_init(struct vop *vop)
 
 		vop_win->data = win_data;
 		vop_win->vop = vop;
-		vop_win->yuv2yuv_data = &vop_data->win_yuv2yuv[i];
+
+		if (vop_data->win_yuv2yuv)
+			vop_win->yuv2yuv_data = &vop_data->win_yuv2yuv[i];
 	}
 }