drm/mm: fix dump table BUG

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

helpers and iterators for hole handling have been introduced with some
debug BUG_ONs sprinkled over. Unfortunately this broke the mm dumper
which unconditionally tried to compute the size of the very first
hole.

While at it unify the code a bit with the hole dumping in the loop.

v2: Extract a hole dump helper.

Reported-by: Christopher Harvey <charvey@matrox.com>
Cc: Christopher Harvey <charvey@matrox.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>