William Havu is keeping cool on a sweltering summer afternoon, surrounded by the sleek sculptures and oil paintings in his white-walled eponymous gallery at 1040 Cherokee Street. It’s worlds away from where he was 56 years ago to the month, eating dinner with Jimi Hendrix and his bandmates at his Dupont Circle apartment in Washington, D.C., then drumming for Hendrix at a show the next night…