Is eye colour of Drosophila a polygenic trait or an X-linked trait?
Polygenic and X-linked are not mutually exclusive; a trait can be polygenic, X-linked, neither, or both.

Polygenic traits are those affected by many genes. According to Conner and Hartl, "Traits that are affected by many gene loci are often called polygenic traits." For example, childhood intelligence is affected by many genes throughout the genome. Monogenic traits are relatively rare, but are linked to some diseases, and occur when a single gene affects the trait; most traits are affected by multiple genes.

X-linked traits are affected by genes (only) situated on the X. If a trait is monogenic, it is very easy to describe it as being, or not being, X-linked. If the gene is on the X, it is X-linked. If it is on the Y, it is Y-linked. If it is in the autosomes, it is autosomally linked. If all of the genes affecting a trait are found on the X then that trait is X-linked. A gene on the X is X-linked, but I would only call a trait X-linked if all genes were on the X. If there was a mix of X-linked and autosomal genetic variation I would describe the trait as having some X-linked genes.

For polygenic traits it is less likely that the genes would be all X-linked, so it is less likely that the trait is X-linked. Imagine a trait affected by 10 genes, and those 10 genes are randomly spread across the genome. It is unlikely that all 10 would be on the X. However, if some were on the X, then we could describe the trait as having X-linked variation. Many highly polygenic traits will have some X-linked variation. 

Having more genes affecting the trait further increases the probability that some genes will be on the sex chromosomes. For example, lifespan is expected to be a highly polygenic trait, and genetic variation in lifepsan can be attributed to the Y chromosome in Drosophila melanogaster, despite the Y carrying just a tiny fraction of the genes. 

Many eye colour traits in Drosophila can be attributed to single genes, and White in the image below is X-linked. However, one can also produce a white-eyed fly using multiple genes; in this paper one of the lines used has white eyes as a result of combining genes (T(2;3) rdgC st pp bwD) which are on the autosomes.