Having a consistent pointer in the drvdata will allow the next patch to
make use of the drvdata from some of the core code helpers.

Use a WARN_ON inside vfio_pci_core_register_device() to detect drivers
that miss this.
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/1-v4-c841817a0349+8f-vfio_get_from_dev_jgg@nvidia.comSigned-off-by: Alex Williamson <alex.williamson@redhat.com>