By "free agency" Mormons refer to a variety of concepts, sometimes one at a time, sometimes multiple. The first is the concept of libertarian free will. This type of free agency would be violated if God sovereignly predestined our choices, or if our own spiritual nature made our choices entirely predictable.
Another is a concept of liberty to carry out in action what one so desires. This type of free agency is violated when one desires or prefers or wills to do something but is not given the freedom to actually carry it out.
Another is to be provided with sufficient information to make an informed choice. This type of free agency is violated when important information is withheld from a person who would use it in a significant way in decision-making.
Mormonism teaches that in the pre-existence Satan proposed a plan of life that excluded free agency (which most likely means libertarian free will in this case), and that Jesus counter-proposed that we be given free agency to prove ourselves worthy, and progress unto godhood.
Mormonism teaches that our agency was in a way enhanced at The Fall.