drm/i915: Add some more registers to error state

Chris:
Do we also want to capture?
  GAC_ECO_BITS /* gen6,7 */
  GAM_ECOCHK /* gen6,7 */
  GAB_CTL /* gen6 */
  GFX_MODE /* gen6 */

Requested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>