drm/imx: ipuv3-plane: decouple zpos from plane type

drm/imx: ipuv3-plane: decouple zpos from plane type

The overlay plane may be placed over or under the primary plane. Using
zpos to determine, if the plane is the primary or overlay plane is not
valid anymore.

Use the plane type for determining the name of the plane in the error
message.

Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://patch.msgid.link/20260219-drm-imx-underlay-plane-v2-1-26ef829c5483@pengutronix.de