Greer Garson, the actress who epitomized a noble, wise and courageous wife in some of the sleekest and most sentimental American movies of the 1940's, died yesterday morning at Presbyterian Hospital in Dallas. She was 92. Miss Garson, who had a history of heart problems, had lived at the long-term-care hospital for the last three years, according to Ann Harper, a spokeswoman at the hospital. Greer Garson was born on Sept. 29, 1903, in County Down, Northern Ireland.