drm/i915: Check for an invalid seqno before __i915_gem_request_started

__i915_gem_request_started() asserts that the seqno is valid, but
i915_spin_request() was not checking before querying whether the request
had started.
Reported-by: Michał Winiarski <michal.winiarski@intel.com>
Fixes: 754c9fd5 ("drm/i915: Protect the request->global_seqno with the engine->timeline lock")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170308142238.22994-1-chris@chris-wilson.co.ukReviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/i915_gem_request.h

@@ -348,6 +348,9 @@ static inline bool i915_spin_request(const struct drm_i915_gem_request *request,
 	u32 seqno;
 
 	seqno = i915_gem_request_global_seqno(request);
+	if (!seqno)
+		return 0;
+
 	return (__i915_gem_request_started(request, seqno) &&
 		__i915_spin_request(request, seqno, state, timeout_us));
 }