drm/i915/selftests: Use full release for local ppgtt allocation

We can now use the full release mechanism (i915_ppgtt_put) for our local
ppgtt allocation in igt_ppgtt_alloc.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180705065653.20449-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/i915_gem_gtt.c

@@ -202,9 +202,8 @@ static int igt_ppgtt_alloc(void *arg)
 
 err_ppgtt_cleanup:
 	mutex_lock(&dev_priv->drm.struct_mutex);
-	ppgtt->vm.cleanup(&ppgtt->vm);
+	i915_ppgtt_put(ppgtt);
 	mutex_unlock(&dev_priv->drm.struct_mutex);
-	kfree(ppgtt);
 	return err;
 }