By Foo Yun Chee BRUSSELS (Reuters) -Microsoft may have to offer concessions to address EU antitrust concerns about its $69 billion bid for “Call of Duty” maker Activision Blizzard after regulators opened a full-scale investigation on Tuesday and warned about the impact of the deal. The U.S. software company, which announced the deal in January, is betting Activision’s stable of games will help it compete better with leaders Tencent and Sony, with the latter critical of the deal. “The Commission’s preliminary investigation shows that the transaction may significantly reduce competition on the m…