The Furyans are said to be a highly spiritual (warrior) race. The dream was a vision. We do know though that Riddick also carries repressed memories.
From Wikia:
While Riddick retained memories of massacre on Furya, these memories were repressed and forgotten, likely due to his extreme youth at the time. Instead, he believed his mother attempted to kill him at birth by strangling him with his umbilical cord and dumping him in a liquour store trash bin, likely the story contained in his records and just as likely told to him repeatedly by those with little care for the child Riddick's sense of self worth.
As Riddick grew older he began recieving [sic] periodic visions of a Furyan named Shirah; visions which he believed to be signs of mental instability. It would not be until his encounter with the Necromongers that his repressed memories would finally escape their imprisonment and he would realize that he had a homeworld and a tragic past that he had never been consciously aware of.
A lot of the backstory is from the Riddick games.
