drm/i915/gvt: remove module refcounting in intel_gvt_{,un}register_hypervisor

THIS_MODULE always is reference when a symbol called by it is used, so
don't bother with the additional reference.

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/20220411141403.86980-2-hch@lst.de
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>