drm/imx: ipuv3-plane: support underlay plane

drm/imx: ipuv3-plane: support underlay plane

The IPUv3 overlay plane may be placed over or under the primary plane.

Set the zpos of the primary to an immutable position of 1 to have the
possibility to place the other plane underneath it.

Set the zpos of other planes (the overlay plane) to a mutable value
between 0 (the lowest possible value of a zpos) and directly above the
primary plane with the latter being the default.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://patch.msgid.link/20260219-drm-imx-underlay-plane-v2-2-26ef829c5483@pengutronix.de