drm/i915: Use lockless object free

Having moved the locked phase of freeing an object to a separate worker,
we can now declare to the core that we only need the unlocked variant of
driver->gem_free_object, and can use the simple unreference internally.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161028125858.23563-20-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.c

@@ -2575,7 +2575,7 @@ static struct drm_driver driver = {
 	.set_busid = drm_pci_set_busid,
 
 	.gem_close_object = i915_gem_close_object,
-	.gem_free_object = i915_gem_free_object,
+	.gem_free_object_unlocked = i915_gem_free_object,
 	.gem_vm_ops = &i915_gem_vm_ops,
 
 	.prime_handle_to_fd = drm_gem_prime_handle_to_fd,