vfio/mdev: turn mdev_init into a subsys_initcall

vfio/mdev: turn mdev_init into a subsys_initcall

Without this setups with buіlt-in mdev and mdev-drivers fail to
register like this:

[1.903149] Driver 'intel_vgpu_mdev' was unable to register with bus_type 'mdev' because the bus was not initialized.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20210726143524.155779-2-hch@lst.de
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>