With a win over the Spurs, the Mavs would ensure to play them in the 1st round since the Blazers own the tiebreaker over the Spurs and the Mavs themselves are up at least 1 game over everybody else. With a loss, a matchup with the Blazers could still happen. LA and Oklahoma City are already confirmed as the first matchup.

To recap:
Dallas has clinched the SW division

Dallas can clinch the 2nd seed by:
1) beating San Antonio (and then playing them guaranteed)
2) losing to San Antonio, but Utah losing at least once
3) losing to San Antonio, but having Denver beat Phoenix

Otherwise, Dallas will finish 3rd