staging: omapdrm: Expose the OMAP Z-Order property through DRM

Added support for zorder changes through DRM plane properties

Signed-off-by: Andre Renaud <andre@bluewatersys.com>
Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>