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.uk


Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>