Why did Ya'akov see fit to mention his death here?
Seforno: He was substantiating the gift of a double portion that he had just given Yosef at the time of death. 1
Rashbam: He was reassuring Yosef that after his death, Hashem would have mercy on Yisrael, and that He would return Yisrael to Eretz Yisrael (as the Pasuk goes on to relate), after the four hundred years that He told Avraham had expired.
Since a gift given at the time of death takes immediate effect.