drm/omap: Use normalized zpos for plane placement

Planes with identical zpos value will result undefined behavior:
disappearing planes, screen flickering and it is not supported by the
hardware.

Use normalized zpos to make sure that we don't encounter invalid
configuration.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
CC: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180321102029.15248-7-peter.ujfalusi@ti.com