If we agree that modernism in poetry can be characterized generally by the poet's alienation from his society, the avoidance of naturalistic representation and the use of fragmentary images, often in odd juxtaposition, then I think we can argue that Yeats's "The Second Coming" is an example of modernism. Whether the poem exhibits "post-modernism" is not as clear.
Clearly, Yeats is describing a world that has gone completely out of control. Even something as natural as the control of a falcon by the falconer has gone awry: the falcon, rather than spiraling down either to its prey or to the falconer's arm, is spiraling outward and is no longer under any control because he cannot even hear the falconer's whistles.
Then, we have a series of images in odd conjunction, which is characteristic of modernism: a bloody tide spreading itself; the drowning of innocence; and the greatest image of a world turned upside down--good people who have lost their ability to believe in anything, and bad people who believe passionately in very bad things.
One aspect generally thought to illustrate modernism in poetry is the poet's "cultural dislocation," the sense that normalcy is no longer the norm, that society is beginning to implode. Yeats's second stanza expresses this aspect fully. He imagines a monstrous being from the desert, "a shape with lion body and the head of a man" (clearly, the Sphinx), is slowly making its way to one of the most culturally significant places on earth: the beast "slouches towards Bethlehem to be born." This is the ultimate expression of the poet's despair, an unmistakable aspect of modernism.
If we take post-modernism to be essentially a continuation of modernism's alienation from society and view of life as an unconnected series of fragments, then "A Second Coming" exhibits those elements. Keep in mind, however, that critics are still trying to decide exactly what post-modernism is, and I don't think it's useful to impute post-modernism to this poem. It is modern enough.