drm: rcar: use generic code for managing zpos plane property

version 6:
rebased patch on top rcar-du changes for zpos

version 4:
fix null pointer issue while setting zpos in plane reset function

This patch replaces zpos property handling custom code in rcar DRM
driver with calls to generic DRM code.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>