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