drm/i915/gvt: move the gvt code into kvmgt.ko

Instead of having an option to build the gvt code into the main i915
module, just move it into the kvmgt.ko module.  This only requires
a new struct with three entries that the KVMGT modules needs to register
with the main i915 module, and a proper list of GVT-enabled devices
instead of global device pointer.

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/20220411141403.86980-7-hch@lst.de
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>