drm/i915: Convert remaining debugfs iterators over rings to for_each_ring()

For code consolidation and future maintainability.

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