drm/i915: Assert we do not try to wait on an invalid seqno

We should never insert the invalid seqno into the wait tree, so assert
we do not.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180102192500.20364-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_breadcrumbs.c

@@ -396,6 +396,8 @@ static bool __intel_engine_add_wait(struct intel_engine_cs *engine,
 	bool first, armed;
 	u32 seqno;
 
+	GEM_BUG_ON(!wait->seqno);
+
 	/* Insert the request into the retirement ordered list
 	 * of waiters by walking the rbtree. If we are the oldest
 	 * seqno in the tree (the first to be retired), then