drm/i915: Do no use kfree() to free a kmem_cache_alloc() return value

Along the eb_lookup_vmas() error path, the return value from
kmem_cache_alloc() was freed using kfree(). Fix it to use the proper
kmem_cache_free() instead.

Fixes: d1b48c1e

 ("drm/i915: Replace execbuf vma ht with an idr")
Signed-off-by: Xidong Wang <wangxidong_97@163.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: <stable@vger.kernel.org> # v4.14+
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180404093824.9313-1-chris@chris-wilson.co.uk
(cherry picked from commit 6be1187d

)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>