In the 1993 film "Gettysburg," a drunken confederate general is heard to say "we should have freed the slaves, then seceded." Was this a popular viewpoint?

That depends on what you mean. Do you mean, did any politician or leader seriously consider freeing the slaves before secession? The answer there is no. Simply read the various ordinances of secession. They are very clear that Northern interference in slavery, or the threat of such, was the deciding factor in their secession. It wouldn't have made any sense to free the slaves.

If you mean, did any Confederate generals value independence more than slavery by that point in the war? Then the answer is yes. Men like Pat Cleburne suggested freeing the slaves as a war necessity. They were strongly opposed up and down the ranks. The desperate situation of the war, and the implications for possible European intervention in the conflict, led the Confederate Congress to pass an Emancipation bill in March, 1865. By a single vote, mind you. It was too little, too late.

Top Sources: Confederate Emancipation and The Fall of the House of Dixie by Bruce Levine

EDIT: OK, People are going to keep commenting, so I should add some clarification. No, the bill passed by the Confederate Congress on March 13 (by just 40-37 and 9-8) did not provide for widespread emancipation. It was targeted at slave soldiers who would be freed on the terms of their service. Given that it passed only 27 days before Appomattox Courthouse, we have no idea if it would have led to a more general emancipation. It certainly undermined the entire white supremacist concept that was a fundamental part of Confederate identity. As Howell Cobb, a one time Speaker of the US House and founding president of the Confederate Congress, said, "You cannot make soldiers of slaves, or slaves of soldiers. The day you make a soldier of them is the beginning of the end of the Revolution. And if slaves seem good soldiers, then our whole theory of slavery is wrong."