Washington Mutual management late last week warned investors that a change in market fundamentals will put increasing pressure on the company’s mortgage banking operations and create a “less favorable” outlook for earnings. In a presentation at an investor conference sponsored by Sanford C. Bernstein & Co., WaMu Chairman and CEO Kerry Killinger said 2004 is shaping up as a “transition year from the extraordinary mortgage banking gains of 2002 and 2003 into a much