drm: Use default dma_fence hooks where possible for null syncobj

Both the .enable_signaling and .release of the null syncobj fence
can be replaced by the default callbacks for a small reduction in code
size. In particular the default callback for .release was changed in
commit e28bd101 ("drm: rename null fence to stub fence in syncobj v2")
which neglected its RCU protection.

Fixes: e28bd101 ("drm: rename null fence to stub fence in syncobj v2")
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/20180920200530.2836-1-chris@chris-wilson.co.uk


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>