iommu/vt-d: Defer domain removal if device is assigned to a driver

When the BUS_NOTIFY_DEL_DEVICE event is received the device
might still be attached to a driver. In this case the domain
can't be released as the mappings might still be in use.

Defer the domain removal in this case until we receivce the
BUS_NOTIFY_UNBOUND_DRIVER event.

Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: stable@vger.kernel.org   # v3.15, v3.16
Signed-off-by: Joerg Roedel <jroedel@suse.de>