drm/i915/gt: Move the breadcrumb to the signaler if completed upon cancel

If while we are cancelling the breadcrumb signaling, we find that the
request is already completed, move it to the irq signaler and let it be
signaled.

v2: Tweak reference counting so that we only acquire a new reference on
adding to a signal list, as opposed to a hidden i915_request_put of the
caller's reference.

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/20201126140407.31952-5-chris@chris-wilson.co.uk