drm/i915: Use the .release hook to drop the stolen drm_mm tracking

Now that we have a release hook into i915_gem_object_free, we can move
the explicit call to the internal stolen function and hook it up
throught the callback instead.

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