From the Rulebook under "‘Before the Game Begins’ and ‘At the End of the Game'"
During your game, you may encounter rules that say that an action or event happens ‘before the game begins’. Examples of such events include generating Warlord Traits and psychic powers. These are always resolved before the armies deploy for battle.
This quote seems to suggest to me that there is no order of operations before the game begins, everything is simply classified as "before the game begins", so order does not matter.
Since Warlord trait generation says "immediately before deploying the first model" I would say you roll psychic powers first if you're worried about order, but if there is a chance to roll a power which affects psychic power generation then common sense would suggest rolling the warlord trait first as that is very clearly the intention.