lavfi/vulkan: use av_get_random_seed instead of rand

lavfi/vulkan: use av_get_random_seed instead of rand

We need at least a few bits of entropy to determine the start index of each
queue, in order to let filters run in parallel as much as possible, and
rand() is not thread safe and disrupts any external API's usage of rand,
so instead replace it with av_get_random_seed.
While it has more overhead than rand, we only run it once per filter upon init.