wsi/display: Rework wsi_fence a bit

wsi/display: Rework wsi_fence a bit

Get rid of most of the guts of the base class and just leave it as a
vtable.  We can also drop some of wsi_display_fence.  One functional
change here is that we're now using VK_SYSTEM_ALLOCATION_SCOPE_INSTANCE
which is more correct anyway because, thanks to the funky reference
counting we do with destroyed and event_received, its lifetime is tied
to the physical device, at best.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13427>