It took a while, but even Democratic activists have tired of "waiting for Mario"--Cuomo, that is. New York's governor says he isn't seeking the White House in 1992 but has refused to rule out a run. At the September meeting of Democratic state chiefs, New York Chairman John Marino got a frosty reception when he said party leaders should give reluctant candidates until Thanksgiving to declare. Even national party Chairman Ron Brown, long close to Cuomo, is losing patience. "I would hope the field would solidify sooner so Democrats know who our candidates are," Brown says.