drm: plane: Verify that no or all planes have a zpos property

The zpos property is used by userspace to sort the order of planes.
While the property is not mandatory for drivers to implement, mixing
planes with and without zpos confuses userspace, and shall not be
allowed. Clarify this in the documentation and warn at runtime if the
drivers mixes planes with and without zpos properties.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>