While there is a Wikipedia entry for "graphist" (given anyone can add or edit Wikipedia), I would state that it's not a valid English word, at least not American English.
It does not appear in the Meriam-Webster dictionary.
In my (quite lengthy) career, I've never heard anyone use that term. Sounds similar to someone calling a plumber a "plumbist" or a doctor a "doctist". The English language is a living breathing thing though. 5 years ago the use of "prolly" for "probably" would have brought scorn in many situations. Now, its use is understood (although it still grates on me). So... who knows.
I would actually tend to equate the term with someone who makes graphs and charts, not a graphic artist or graphic designer.