drm/i915: Consolidate gem object list dumping

Here we eliminate a few functions in favor of using a single function
to dump from all of the object lists.

Signed-Off-By: Ben Gamari <bgamari@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>