drm/rockchip: fixup comment for gem_free_object_unlocked

Just want to clean out all grep hits. gem_free_object is deprecated.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Sandy Huang <hjc@rock-chips.com>
Cc: "Heiko Stübner" <heiko@sntech.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-rockchip@lists.infradead.org
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180327082356.24516-2-daniel.vetter@ffwll.ch