anv: add missing unlock in error path.

Not going to matter, but be consistent.

Found by coverity

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Fixes: caf41c78 (anv/allocator: Support softpin in the BO cache)
(cherry picked from commit 29a76319)