drm/i915: ivybridge_handle_parity_error should be static

Notice by Fengguang Wu's automatic sparse checker.

Reported-by: Fengguang Wu <wfg@linux.intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>