Spring in New York usually brings a museum survey of new art. Such shows don't have to be ''great''; they seldom are. But if they're good, they deliver a hint of newness and promise that the commercial art world, month by month, can only sporadically muster. Sometimes a Whitney Biennial fills the bill; last year it was ''Greater New York.'' This spring ''Freestyle,'' at the Studio Museum in Harlem, is the showcase to see.