drm/virtio: use virtio_max_dma_size

We must make sure our scatterlist segments are not too big, otherwise
we might see swiotlb failures (happens with sev, also reproducable with
swiotlb=force).

Suggested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190821111210.27165-1-kraxel@redhat.com