Let's make sure we give the driver a valid workqueue.

While at it, also make sure to call destroy_workqueue() only if the
workqueue is a valid one. That is necessary because xe_device_destroy()
is indirectly called as part of the cleanup process of a failed
xe_device_create().
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20230518215651.502159-3-gustavo.sousa@intel.comSigned-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>