drm/i915: fix reference counting in i915_gem_create

This function is called without the dev->struct_mutex held, hence we
need to use the _unlocked unreference variants.

As soon as the object is registered userspace can sneak in here with a
gem_close ioctl call, so the object can (and with my new evil tests
actually does) get the final unreference in this place. The lack of
locking then results in hilarity and some good leakage.

To fix this we simply need to revert

Chris Wilson <chris@chris-wilson.co.uk>

v2: We need to make the trace call _before_ we drop our ref - the
object might very well be gone by then already.

v3: Just revert the original patch as suggested by Chris Wilson.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Remove the added white line again to tighten the return
block, requested by Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>