drm/i915/gem: Silence sparse for RCU protection inside the constructor

drm/i915/gem: Silence sparse for RCU protection inside the constructor

Inside the constructor, while cloning, we need to replace the
dst->engines. Having forgotten that dst->engines is marked as RCU
protected, we need to add the appropriate annotations to make sparse
happy.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191114225736.616885-5-chris@chris-wilson.co.uk