Yes, Dirty Harry Callahan is extremely politically incorrect. And no, the movies are certainly not Eastwood's best performance. But fascist? No.
But one has to recognize that Dirty Harry became an icon nevertheless, an instantly recognizable reference point of the age. IIRC the motto Terry Pratchett put above the gate of his Nightwatch station was FABRICATE DIEM MEUM PVNC -- and he could count on it that the joke would be understood even by people who knew no Latin. (Question for Pratchett fans -- is Sam Vimes based on Harry Callahan? I think he is, but Vimes keeps his dark forces under control.)
It's a dark fantasy. But it's not a fascist fantasy. People granted Dirty Harry iconic status and felt affinity for this rather nasty character, because Harry Callahan is very much an individual character -- cartoonish, but unique. In a fascist fantasy, individualism has no place. It's impossible to imagine Dirty Harry in a black uniform and jackboots. If only because he is far too undisciplined and refuses to fit in and merge into the mass. Sure, he is somewhere to the right of Timur the Lame, but one can be right-wing, even very far right-wing, without being fascist. Dirty Harry believes in violence. Fascists believe in organized violence.
Nasty? Yes. But this has to be seen in the context of the real America, which gruesomely tortures people to death on the electric chair, instead of some ideological fairly-tale land in which people cheerfully respect each other constitutional rights. For better or worse, Dirty Harry actually embodies real American values. Nobody is obliged to like them, but shutting your eyes doesn't really help.