drm/i915: Unref old_obj on get_fence_reg() error path

Remember to release the local reference if we fail to wait on
the rendering.

(Also whilst in the vicinity add some whitespace so that the phasing of
the operations is clearer.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>