anv: Use DRM sync objects to back fences whenever possible

anv: Use DRM sync objects to back fences whenever possible

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>