drm/i915/gvt: remove the vgpu->released and its sanity check

drm/i915/gvt: remove the vgpu->released and its sanity check

The life cycle of a vGPU, which is represented by a vfio_device, has been
managed by the VFIO core logic. Remove the vgpu->released, which was used
for a sanity check on the removal path of the vGPU instance. The sanity
check has already been covered in the VFIO core logic.

Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: intel-gvt-dev@lists.freedesktop.org
Suggested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20221104145652.1570-1-zhi.a.wang@intel.com
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>