Add explicit check for is_liveblog_post in display_post_state filter

The is_liveblog_post() method actually just uses get_liveblog_state(), but this is more clear.

See https://github.com/x-team/wp-liveblog/commit/68e2c7bd58f3ca952558ac074f48ba5b4f3e118b#commitcomment-3570526