drm/i915: Allow init_breadcrumbs to be used from irq context

In order to support engine reset from irq (timer) context, we need to be
able to re-initialise the breadcrumbs. So we need to promote the plain
spin_lock_irq to a safe spin_lock_irqsave.

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/20180518090212.5349-3-chris@chris-wilson.co.uk