No, CNN reported that the President and Trump had been briefed on the dossier. They did not report its contents, and also stated (as Buzzfeed did tbf) that the contents were unverified. This (the briefing, not the contents) has subsequently been proved accurate.
The President and Trump *being briefed* on a document CLAIMING (not proving, CLAIMING) that Trump is vulnerable to Russian blackmail isn't fake news. It's just news.
Buzzfeed then dumped the entire contents online, in a move which you can agree or disagree with (though it's not an unheard of move - run with what you've got, and see if it shakes anything more concrete loose), but it doesn't make what CNN reported "fake news" or anything close to it. Nor does it excuse the man who's supposed to be leading the country in a few days, acting like a petulant child in his first press conference in 167 days.