drm/i915: fix up error path leak in i915_cmdbuffer

Commit 201361a5

 introduces a leak when unwinding on error.  Reorder
unwind, and eliminate leak.

Cc: Eric Anholt <eric@anholt.net>
Cc: Keith Packard <keithp@keithp.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
[anholt: fixed uninit variable use introduced in original patch]
Signed-off-by: Eric Anholt <eric@anholt.net>