drm/rockchip: vop2: Switch impossible pos conditional to WARN_ON

drm/rockchip: vop2: Switch impossible pos conditional to WARN_ON

We already clip the plane to the display bounds in atomic_check, and
ensure that it is sufficiently sized. Instead of trying to catch this
and adjust for it in atomic_update, just assert that atomic_check has
done its job.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patch.msgid.link/20251215-vop2-atomic-fixups-v5-2-83463c075a8d@collabora.com