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

Cc: stable@kernel.org
Signed-off-by: NTakashi Iwai <tiwai@suse.de>
[danvet: added cc stable]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>