How and why did T'Challa have the Black Panther power already?

Marvel published a prequel comic book to Black Panther, called Black Panther Prelude, that details the story of T'Challa becoming the Black Panther. This actually occurred very early in the MCU timeline, roughly at the same time that Tony Stark was outing himself as Iron Man. His father chose to step down and pass on the mantle of Black Panther when it became clear he was too old to continue and his son was ready and able.

So, by the time of Captain America: Civil War, T'Challa had already taken the heart-shaped herb and been the Black Panther for 7-8 years.

Read the Black Panther Prelude comic.

In comics, T'Challa didn't have the powers of heart shaped herb. Instead when he returned to Wakanda from his studies in US, he was sent to find heart shaped heart, but he failed.

He defeated the then Black Panther and took heart shaped herb afterwards.So on Earth 616, he didn't have powers before becoming the king.

But in Marvel Cinematic Universe (Earth 199999), things are different. He had powers before he became the king.

The reason for this is the fact that he was the apparent successor to the throne.

At one point in the movie, I remember T'Chaka reminds him that he has been preparing for this role (The King) for his entire life.

So it could have been that he was given the herb for the training of the future king of Wakanda.

While the king is often the black panther, and the black panther is often the king, we know from Avenger's: Civil War that T'Challa was black panther while his father was the king. If his father had the advantage of the vibranium flowers he likely wouldn't have died from that attack, therefore we can conclude that he, for an unknown as yet reason, chose to remove his powers, and chose his son to gain them prior to that movie.

For any of a variety of reasons a king might choose another to become Wakanda's Black Panther, there is no hard and fast requirement in the MCU universe (yet) that means the king and black panther are always one and the same. While in the Black Panther movie it appears that it's expected, the ceremony to fight for leadership only ever removes the power and a separate ceremony gives it - therefore it's possible for a king to choose to refuse the power and role.

Outside the MCU there are other Back Panthers who weren't kings, as well as two Black Panthers at once, but the MCU hasn't fleshed out as much of the Black Panther mythos.