oar-properties: fix gpu_idx computation

Quickfix to be able to generate oar properties until I provide a proper patch for https://intranet.grid5000.fr/bugzilla/show_bug.cgi?id=15068.

It doesn't fix the fact that GPUs are not using all processors, but it does fix how the processor is split when the number of cores cannot be divided evenly. In the case of abacus9 there are 4 GPUs sharing a 10 cores processor, so 2 of them will have 3 cores while the other 2 will have 2.