drm/i915: Show pin mapped counts and sizes in debugfs

Show a total and purgeable number of pin mapped objects
and their total and purgeable size.

Example output (new stat prefixed with a star):

  # cat i915_gem_objects
  19920 objects, 289243136 bytes
  19920 [18466] objects, 288714752 [267911168] bytes in gtt
    0 [0] active objects, 0 [0] bytes
    19917 [18466] inactive objects, 288714752 [267911168] bytes
  0 unbound objects, 0 bytes
  0 purgeable objects, 0 bytes
  1 pinned mappable objects, 3145728 bytes
  0 fault mappable objects, 0 bytes
* 19914 [0] pin mapped objects, 285560832 [0] bytes [purgeable]
  4294967296 [268435456] gtt total

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1460716493-27826-1-git-send-email-tvrtko.ursulin@linux.intel.com