drm: More debug info for fb leaks in mode_config_cleanup

We're spotting this very rarely in CI, but have no idea. Let's add
more debug info about what's going on here.

References: https://bugs.freedesktop.org/show_bug.cgi?id=102707


Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171207144925.14191-1-daniel.vetter@ffwll.ch