drm: rcar-du: Fix plane state free in plane reset handler

The plane reset handler frees the plane state and allocates a new
default state, but when doing so attempt to free the plane state using
the base plane state pointer instead of casting it to the
driver-specific state object that has been allocated. Fix it by using
the rcar_du_plane_atomic_destroy_state() function to destroy the plane
state instead of duplicating the code.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>