drm: rcar-du: Fix compilation warning

Commit d63c25e4 ("drm: rcar-du: Use generic
drm_connector_register_all() helper") left an unused local variable
behind. Remove it.

Fixes: d63c25e4 ("drm: rcar-du: Use generic drm_connector_register_all() helper")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>