venus: optimize mutable state restore

Summary:
1. skip zero count
2. no need to check last binding count on the restore path
3. flatten the helper to avoid a 2nd pass in free_descriptors

Signed-off-by: Yiwei Zhang <zzyiwei@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28563>
1 file changed