drm/i915: Add -EIO to the list of known errors for __wait_seqno

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: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>