The fixed date elections act did not at all deal with the role of the Prime Minister. The purpose of the first section of this act was to amend section 51.6 of the Canada Elections Act, to which the following clarification was added:
56.1 (1) Nothing in this section affects the powers of the Governor General [...]
This gave the Prime Minister the ability to advise the Governor General with regard to the calling of an election. This is indeed a reservation set out in the act:
[...] including the power to dissolve Parliament at the Governor General's discretion.
The Governor General acts in accordance with the advice of the Prime Minister. If this act stated expressly that the Prime Minister could not give advice to the Governor General other than every four years, or something of that nature, it would be a different story.