drm/i915/gvt/kvmgt: prevent double-release of vgpu

The release action might be triggered from either user's closing
mdev or the detaching event of kvm and vfio_group, so this patch
introduces an atomic to prevent double-release.

Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>