When an effect is granting an ability to a creature, and that granted ability uses the phrase “this creature” (or similar), it's talking about the creature that was granted the ability.
At no point does the ability granted by
Dragon Throne of Tarkir care what, if anything, the Dragon Throne is currently attached to; moving the Throne after activation will have no effect whatsoever on the ability on the stack. If the ability granted by the Dragon Throne was trying to check the power of whatever creature the Dragon Throne was attached to on resolution, it would say that.