drm: omapdrm: Add zpos property

Create a standard zpos property for every plane as an alias to the
omapdrm-specific zorder property. Unlike the zorder property that has to
be instantiated for both planes and CRTCs due to backward compatibility,
the zpos property is only instantiated for planes. When userspace will
have switched to the zpos property the zorder property will be removed.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>