This prevents a WARN introduced with

  commit de2b9985
  Author: Daniel Vetter <daniel.vetter@ffwll.ch>
  Date:   Wed Jul 4 22:52:50 2012 +0200

      drm/i915: don't return a spurious -EIO from intel_ring_begin
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>