drm/i915: Print how many objects are shared in per-process stats

The point of this measure is to gauge why a process has a lot of gem
objects in uses and why. Especially for compositors it's interesting
to know whether it's a leak of private objects or just a lot of use
from buffers shared with clients.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Add a bit of commit message flesh to address Ben's comment.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>