drm/i915/gvt: remove the extra vfio_device refcounting for dmabufs

All the dmabufs are torn down when th VGPU is released, so there is
no need for extra refcounting here.

Based on an patch from Jason Gunthorpe.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20220411141403.86980-25-hch@lst.de


Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>