drm/i915/gt: Save/restore interrupts around breadcrumb disable

Stop assuming we only get called with irqs-on for disarming the
breadcrumbs, and do a full save/restore spin_lock_irq.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190813132916.20382-2-chris@chris-wilson.co.uk