drm/i915: Don't print header in error state for non-existing CS

This goes back to

commit 362b8af7


Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Thu Jan 30 00:19:38 2014 -0800

    drm/i915: Move per ring error state to ring_error

Spotted while reading error states.

Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>