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

This prevents a WARN introduced with

  commit de2b998552c1534e87bfbc51ec5734b02bc89020
  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>

drivers/gpu/drm/i915/i915_gem.c

@@ -2003,6 +2003,7 @@ static int __wait_seqno(struct intel_ring_buffer *ring, u32 seqno,
 	}
 
 	switch (end) {
+	case -EIO:
 	case -EAGAIN: /* Wedged */
 	case -ERESTARTSYS: /* Signal */
 		return (int)end;