vfio/mdev: Expose mdev_get/put_parent to mdev_private.h

The next patch will use these in mdev_sysfs.c

While here remove the now dead code checks for NULL, a mdev_type can never
have a NULL parent.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Message-Id: <6-v2-d36939638fc6+d54-vfio2_jgg@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>