drm/i915: Add information on pinning and fencing to the i915 list debug.

This was inspired by a patch by Chris Wilson, though none of it applied in any
way due to the debugfs work and I decided to change the formatting of the
new information anyway.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>