drm/virtio: drop prime import/export callbacks

Also set prime_handle_to_fd and prime_fd_to_handle to NULL,
so drm will not advertive DRM_PRIME_CAP_{IMPORT,EXPORT} to
userspace.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190110111545.26768-1-kraxel@redhat.com