drm/mm: fix debug table BUG

In

commit 3a359f0b
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sat Apr 20 12:08:11 2013 +0200

    drm/mm: fix dump table BUG

I've failed to fix both instances of the regression introduced in

commit 9e8944ab
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu Nov 15 11:32:17 2012 +0000

   drm: Introduce an iterator over holes in the drm_mm range manager

Patch this up in the same way by extracting the hole debug logic
into it's own function, since that'll also clarify the logic a bit.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>