drm/virtio: make resource id workaround runtime switchable.

Also update the comment with a reference to the virglrenderer fix.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190822102614.18164-1-kraxel@redhat.com