vfio/gvt: Make DRM_I915_GVT depend on VFIO_MDEV

At some point there may have been some reason for this weird split in this
driver, but today only the VFIO side is actually implemented.

However, it got messed up at some point and mdev code was put in gvt.c and
is pretending to be "generic" by masquerading as some generic attribute list:

   static MDEV_TYPE_ATTR_RO(description);

But MDEV_TYPE attributes are only usable with mdev_device, nothing else.

Ideally all of this would be moved to kvmgt.c, but it is entangled with
the rest of the "generic" code in an odd way. Thus put in a kconfig
dependency so we don't get randconfig failures when the next patch creates
a link time dependency related to the use of MDEV_TYPE.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Message-Id: <15-v2-d36939638fc6+d54-vfio2_jgg@nvidia.com>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>