drm/i915: Add some sanity checks to unbound tracking

A pair of universally true checks that just need to be put in the right
place depending on where in the patch sequence you go. Note that
i915_gem_object_put_pages_gtt() already gains the
BUG_ON(obj->gtt_space), but on reflection that needed to migrate to
put_pages().

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>