If one knows that what has to happen will happen, by the will of Supreme, then why do we protect our dharma? We do not control the results (fruits). By the will of Supreme, if dharma has to stay, it will, and if it does not have to, it will not. Then why should we try to protect our dharma when the results are not in our hands?
By considering the point "Everything is already planned", then literally everything is planned including our fight against Adharma. Means we cannot stop ourselves in playing our part which is planned for us.
Now coming to the scriptural basis, Bhagvad Gita in Chapter three Karma Yoga section says that everyone is forced to work helplessly according to their qualities. Even no one can maintain their physical body without work.
Bg 3.3 — The Supreme Personality of Godhead said: O sinless Arjuna, I have already explained that there are two classes of men who try to realize the self. Some are inclined to understand it by empirical, philosophical speculation, and others by devotional service.
Bg 3.4 — Not by merely abstaining from work can one achieve freedom from reaction, nor by renunciation alone can one attain perfection.
Bg 3.5 — Everyone is forced to act helplessly according to the qualities he has acquired from the modes of material nature; therefore no one can refrain from doing something, not even for a moment.
Bg 3.6 — One who restrains the senses of action but whose mind dwells on sense objects certainly deludes himself and is called a pretender.
Bg 3.7 — On the other hand, if a sincere person tries to control the active senses by the mind and begins karma-yoga [in Kṛṣṇa consciousness] without attachment, he is by far superior.
Bg 3.8 — Perform your prescribed duty, for doing so is better than not working. One cannot even maintain one’s physical body without work.
Bg 3.9 — Work done as a sacriﬁce for Viṣṇu has to be performed; otherwise work causes bondage in this material world. Therefore, O son of Kuntī, perform your prescribed duties for His satisfaction, and in that way you will always remain free from bondage.