drm/i915: Prevent use of global_seqno=0

We are not allowed to assign rq->global_seqno=0 as it has a special
meaning of "inactive" (not executing on HW).

Fixes: 6faf5916 ("drm/i915: Remove HW semaphores for gen7 inter-engine synchronisation")
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/20190119143024.26971-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_request.c

@@ -343,6 +343,13 @@ static void move_to_timeline(struct i915_request *request,
 	spin_unlock(&request->timeline->lock);
 }
 
+static u32 next_global_seqno(struct i915_timeline *tl)
+{
+	if (!++tl->seqno)
+		++tl->seqno;
+	return tl->seqno;
+}
+
 void __i915_request_submit(struct i915_request *request)
 {
 	struct intel_engine_cs *engine = request->engine;
@@ -359,7 +366,7 @@ void __i915_request_submit(struct i915_request *request)
 
 	GEM_BUG_ON(request->global_seqno);
 
-	seqno = timeline_get_seqno(&engine->timeline);
+	seqno = next_global_seqno(&engine->timeline);
 	GEM_BUG_ON(!seqno);
 	GEM_BUG_ON(intel_engine_signaled(engine, seqno));