In order to implement VK_KHR_external_fence, we need to back our fences
with something that's shareable. Since the kernel wait interface for
sync objects already supports waiting for multiple fences in one go, it
makes anv_WaitForFences much simpler if we only have one type of fence.
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
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