drm/virtio: Use IDAs more efficiently

0-based IDAs are more efficient than any other base.  Convert the
1-based IDAs to be 0-based.

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20181030165352.13065-2-willy@infradead.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>