drm: omapdrm: omap_plane_setup() cannot fail, use WARN

With atomic modesetting, omap_plane_setup()'s return value is ignored as
the functions using it cannot fail. dispc_ovl_setup(), called by
omap_plane_setup(), can fail (but shouldn't).

Instead of returning an error from omap_plane_setup() which gets
ignored, return void and use WARN if dispc_ovl_setup() fails.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>