drm/i915/gt: Defer engine registration until fully initialised

Only add the engine to the available set of uabi engines once it has
been fully initialised and we know we want it in the public set.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michał Wajdeczko <michal.wajdeczko@intel.com>
Cc: Andi Shyti <andi.shyti@intel.com>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107081252.10542-17-chris@chris-wilson.co.uk