drm/i915: opregion_setup: iounmap correct address

In case of an opregion signature mismatch in intel_opregion_setup(),
iounmap the correct address.

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Cc: stable@kernel.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>