drm/i915/gt: Close race between enable_breadcrumbs and cancel_breadcrumbs

If we enable_breadcrumbs for a request while that request is being
removed from HW; we may see that the request is active as we take the
ce->signal_lock and proceed to attach the request to ce->signals.
However, during unsubmission after marking the request as inactive, we
see that the request has not yet been added to ce->signals and so skip
the removal. Pull the check during cancel_breadcrumbs under the same
spinlock as enabling so that we the two tests are consistent in
enable/cancel.

Otherwise, we may insert a request onto ce->signals that we expect should
not be there:

  intel_context_remove_breadcrumbs:488 GEM_BUG_ON(!__i915_request_is_complete(rq))

While updating, we can note that we are always called with
irqs-disabled, due to the engine->active.lock being held at the single
caller, and so remove the irqsave/restore making it symmetric to
enable_breadcrumbs.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2931
Fixes: c18636f7

 ("drm/i915: Remove requirement for holding i915_request.lock for breadcrumbs")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Andi Shyti <andi.shyti@intel.com>
Cc: <stable@vger.kernel.org> # v5.10+
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210119162057.31097-1-chris@chris-wilson.co.uk