drm/i915: Remove i915_gem_ringbuffer_(data|info) from debugfs

This was originally used as an attempt to diagnose GPU hangs, but was
never very reliable and superseded by the i915_error_state capture on
hangcheck. It now lies languishing unused and unwanted.

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