radv: use syncobj for wsi fence

Note: this also fixes vk_object_base_init() not being called for the wsi
fences, now that it goes through radv_CreateFence().

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6846>