drm: rcar-du: Set primary plane zpos immutably at initializing

According to drm_plane_create_zpos_property() function documentation,
all planes zpos range should be set if zpos property is supported.
However, the rcar-du driver didn't set primary plane zpos range. Since
the primary plane's zpos is fixed, set it immutably.

Reported-by: Yoshihito Ogawa <yoshihito.ogawa.kc@renesas.com>
Reported-by: Koji Matsuoka <koji.matsuoka.xm@renesas.com>
Signed-off-by: Tomohito Esaki <etom@igel.co.jp>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
[Turn continue into if ... else ...]
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>