drm/i915: Check that the breadcrumb wasn't disarmed automatically before parking

We will disarm the breadcrumb interrupt if we see a user interrupt
whilst no one is waiting. This may race with the call to
intel_engine_disarm_breadcrumbs() triggering an assert that we aren't
trying to do the same job twice. Prevent this by checking that the irq
is still armed after flushing the interrupt (for the irq spinlock).

Fixes: bcbd5c33a342 ("drm/i915/guc: Always enable the breadcrumbs irq")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171031122235.1395-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/intel_breadcrumbs.c

@@ -219,7 +219,8 @@ void intel_engine_disarm_breadcrumbs(struct intel_engine_cs *engine)
 
 	spin_lock(&b->irq_lock);
 	first = fetch_and_zero(&b->irq_wait);
-	__intel_engine_disarm_breadcrumbs(engine);
+	if (b->irq_armed)
+		__intel_engine_disarm_breadcrumbs(engine);
 	spin_unlock(&b->irq_lock);
 
 	rbtree_postorder_for_each_entry_safe(wait, n, &b->waiters, node) {