drm/virtio: implement prime mmap

Sync gem vm_node.start with ttm vm_node.start,
then we can just call drm_gem_prime_mmap().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190227144441.6755-2-kraxel@redhat.com