staging: mrst: Add overlay Z order support

staging: mrst: Add overlay Z order support

Allow the Z order of the overlay to be selected.

Note that there is something strange going on though. It seems changing
the Z order also affects destination color keying. When Z order is
configured one way destination color keying works, but when the Z order
is flipped destination color keying no longer is effective.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Pauli Nieminen <pauli.nieminen@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>