drm/virtio: Replace IDRs with IDAs

These IDRs were only being used to allocate unique numbers, not to look
up pointers, so they can use the more space-efficient IDA instead.

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20180926160031.15721-2-willy@infradead.org

[ kraxel: resolve conflict ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>