You're worrying about nothing, and trying to go against the engineering design of the card. First of all, the GTX 980 Ti runs a little warmer than other Maxwell cards. The thermal threshold is 95c, which is the point at which it may risk damage.
The other part is that you need to understand how Turbo Boost 2.0 works on Maxwell cards. Turbo Boosts above your core clock speed are governed by a factory preset temperature target, called the Temp Limit in PrecisionX. For your card that temp target is 84c. This is the temperature that your card attempts to seek out and maintain as a part of its normal operation. Above this temperature, your card will dial back on the Turbo Boost clocks and increase fan speeds. Below 84c your card will increase Turbo Boost clocks in order to fill that thermal overhead and provide more performance.
So: 1) You're running at a temperature well below your temperature target, exactly as the card was designed.
2) You're probably running with some nice Turbo Boost clocks and not being limited by hitting your Temp Limit.
3) Messing with the fans is only going to increase noise, in order to attain an arbitrary number that's meaningless in terms of your Turbo Boost algorithms.
The recommendation is to put your fans on Auto and stop worrying as long as your temperatures stay below 85c. Your card is running perfectly as designed and a temperature of 79c on a highly factory overclocked card is actually pretty good.