Individual Mormons did, not the Mormon Church. See the correction at the bottom of this article from The Wall Street Journal.
The bottom of that article (bolding mine):
30-40% of donations from a group that comprises less than 3% of the state (far less in most counties) is pretty impressive, and it’s also inconceivable that the church itself wasn’t instrumental in convincing them to donate. In fact some flat out said it was, including theartistic directorof the California Musical Theatre who resigned when his donations (understandably) caused controversy in a manner roughly equivocal to the regional director of Focus on the Family contributing to Human Rights Campaign. (Maybe he can get a job at the Osmond Brothers Dinner Theatre in Branson, or a job selling Amway where he doesn’t have to interact with “best friends” who he thinks are living in complete opposition to his moral beliefs.)
Celebrity power, maybe. Money, no. Just plain no.
Total Contributions re Proposition 8
To oppose ban on same-sex marriage: $44,103,525
To support ban on same-sex marriage: $38,766,260
Both in-state and out-of-state contributions were higher to oppose the ban.
Source: Los Angeles Times, Feb. 3, 2009.