vfio/gvt: fix DRM_I915_GVT dependency on VFIO_MDEV

The Kconfig dependency is incomplete since DRM_I915_GVT is a 'bool'
symbol that depends on the 'tristate' VFIO_MDEV. This allows a
configuration with VFIO_MDEV=m, DRM_I915_GVT=y and DRM_I915=y that
causes a link failure:

x86_64-linux-ld: drivers/gpu/drm/i915/gvt/gvt.o: in function `available_instances_show':
gvt.c:(.text+0x67a): undefined reference to `mtype_get_parent_dev'
x86_64-linux-ld: gvt.c:(.text+0x6a5): undefined reference to `mtype_get_type_group_id'
x86_64-linux-ld: drivers/gpu/drm/i915/gvt/gvt.o: in function `description_show':
gvt.c:(.text+0x76e): undefined reference to `mtype_get_parent_dev'
x86_64-linux-ld: gvt.c:(.text+0x799): undefined reference to `mtype_get_type_group_id'

Clarify the dependency by specifically disallowing the broken
configuration. If VFIO_MDEV is built-in, it will work, but if
VFIO_MDEV=m, the i915 driver cannot be built-in here.

Fixes: 07e543f4 ("vfio/gvt: Make DRM_I915_GVT depend on VFIO_MDEV")
Fixes: 9169cff1 ("vfio/mdev: Correct the function signatures for the mdev_type_attributes")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Message-Id: <20210422133547.1861063-1-arnd@kernel.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>