How can the hadith "A righteous vision is a part of prophecy" be reconciled with the visions of non-Muslims mentioned in the Qur'an, such as Pharaoh and the king in the time of Joseph?

A truthful dream is a part of prophethood, and it is specific to the righteous believer. As for the disbeliever, he may see a truthful dream, but it is not a part of prophethood or revelation. A righteous dream from a righteous believer is considered a form of revelation because his state is akin to that of the prophets in being honored with a portion of what they were honored with. The disbeliever and the wicked, even if their dreams are sometimes truthful, their dreams are not prophethood or revelation.