A federal judge has upheld a controversial minority hiring plan in Birmingham, Ala., rejecting the Justice Department's contention that the city is discriminating against white employees. U.S. District Judge Sam C. Pointer Jr. said that a 1981 consent decree allows Birmingham to hire and promote blacks and women in municipal jobs over more qualified white male candidates. He said the agreement is a valid defense against charges of discrimination. Birmingham is a majority black city with a long history of discrimination. Before 1974, when the first lawsuit challenging city hiring practices was filed, only two of the Fire Department's 640 employees were black.