The embattled Southern Poverty Law Center, which has won two Academy Awards for its documentaries and has advised numerous TV shows on hate crimes, has lost another top executive. Richard Cohen, who headed the SPLC since 2003, said Friday that a search for his successor is underway. His announcement comes a week after the nonprofit organization fired founder Morris Dees for unspecified misconduct. Cohen said his departure was "in order to give the organization the best…