drm/i915: add accounting for mappable objects in gtt v2

More precisely: For those that _need_ to be mappable. Also add two
BUG_ONs in fault and pin to check the consistency of the mappable
flag.

Changes in v2:
- Add tracking of gtt mappable space (to notice mappable/unmappable
  balancing issues).
- Improve the mappable working set tracking by tracking fault and pin
  separately.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>