drm/i915/gvt: Add missing vfio_unregister_group_dev() call

When converting to directly create the vfio_device the mdev driver has to
put a vfio_register_emulated_iommu_dev() in the probe() and a pairing
vfio_unregister_group_dev() in the remove.

This was missed for gvt, add it.

Cc: stable@vger.kernel.org
Fixes: 978cf586 ("drm/i915/gvt: convert to use vfio_register_emulated_iommu_dev")
Reported-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/0-v1-013609965fe8+9d-vfio_gvt_unregister_jgg@nvidia.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>