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>

diff --cc drivers/gpu/drm/i915/i915_drv.h
Simple merge

diff --cc drivers/gpu/drm/i915/i915_gem.c
Simple merge

diff --cc drivers/gpu/drm/i915/i915_gem_stolen.c
Simple merge