It's perfectly fine in English, even though it may be a little literary in tone. Here, "bent" means "angled" (ie. directed at an angle, usually downwards).

"His eyes were bent fixedly before him" means he was staring at the floor, even as he rocked. Normally one would say "gaze was bent", but, hey, I'm definitely not going to argue with Poe. Maybe he was being a little more visceral or unusual.

I think the phrase “bend one’s eyes” comes from the old belief that the eyes “shoot out beams” to see objects. And “bend” was a common word for pulling the string of your bow and holding it before shooting your arrow, fixing your eye on the target. So to bend one’s eye does not mean to shape it in a curve, but to fix it straight ahead on something: therefore to stare steadily or intently at, to “fix” one’s eyes on, almost as if the were attached to the thing you’re looking at.