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>

drivers/gpu/drm/i915/i915_gem.c

@@ -1378,6 +1378,8 @@ i915_gem_object_put_pages_gtt(struct drm_i915_gem_object *obj)
 	int page_count = obj->base.size / PAGE_SIZE;
 	int ret, i;
 
+	BUG_ON(obj->gtt_space);
+
 	if (obj->pages == NULL)
 		return 0;
 
@@ -2293,6 +2295,8 @@ i915_gem_object_unbind(struct drm_i915_gem_object *obj)
 	if (obj->pin_count)
 		return -EBUSY;
 
+	BUG_ON(obj->pages == NULL);
+
 	ret = i915_gem_object_finish_gpu(obj);
 	if (ret)
 		return ret;