agp/intel: Fix wrong kunmap in i830_cleanup()

Add a missing NULL check and fix the wrong address passed to kunmap()
in i830_cleanup().

Cc: stable@kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[danvet: added cc stable]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>