drm: omapdrm: Store the Z order in the plane state zpos field

The DRM core implements a standard "zpos" property to control planes
ordering. The omapdrm driver implements a similar property named
"zorder". Although we can't switch to DRM core handling of the "zpos"
property for backward compatibility reasons, we can store the zorder
value in the drm_plane_state zpos field, saving us from adding a custom
field to the plane state.

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