drm: sti: use generic zpos for plane

remove private zpos property and use instead the generic new.
zpos range is now fixed per plane type and normalized before
being using in mixer.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>

Cc: Inki Dae <inki.dae@samsung.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: vincent.abriou@st.com
Cc: fabien.dessenne@st.com
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>