By stashing a pointer of who opened the device and keeping a list of
open fd, we can then walk each client and inspect how many objects they
have open. For example,

i915_gem_objects:
1102 objects, 613646336 bytes
663 [662] objects, 468783104 [468750336] bytes in gtt
  37 [37] active objects, 46874624 [46874624] bytes
  626 [625] inactive objects, 421908480 [421875712] bytes
282 unbound objects, 6512640 bytes
85 purgeable objects, 6787072 bytes
28 pinned mappable objects, 3686400 bytes
40 fault mappable objects, 27783168 bytes
2145386496 [536870912] gtt total

Xorg: 43 objects, 32243712 bytes (10223616 active, 16683008 inactive, 4096 unbound)
gnome-shell: 30 objects, 28381184 bytes (0 active, 28336128 inactive, 0 unbound)
xonotic-linux64: 1032 objects, 569933824 bytes (46874624 active, 383545344 inactive, 6508544 unbound)

v2: Use existing drm->filelist as pointed out by Ben.
v3: Not even stashing the task_struct is required as Ben pointed out
    drm_file->pid.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>