drm/i915: Fix object refcount leak on mmappable size limit error path.

I've been seeing memory leaks on my system in the form of large
(300-400MB) GEM objects created by now-dead processes laying around
clogging up memory.  I usually notice when it gets to about 1.2GB of
them.  Hopefully this clears up the issue, but I just found this bug
by inspection.

Signed-off-by: Eric Anholt <eric@anholt.net>
Cc: stable@kernel.org
Signed-off-by: Keith Packard <keithp@keithp.com>