anv: destroy descriptor sets when pool gets reset

anv: destroy descriptor sets when pool gets reset

As stated in Vulkan spec:
   "Resetting a descriptor pool recycles all of the resources from all
    of the descriptor sets allocated from the descriptor pool back to
    the descriptor pool, and the descriptor sets are implicitly freed."

This fixes dEQP-VK.api.descriptor_pool.*

Fixes: 14f6275c92f1 "anv/descriptor_set: add reference counting for..."
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Tested-by: Clayton Craft <clayton.a.craft@intel.com>