venus: properly ignore the sampler for immutable sampler

venus: properly ignore the sampler for immutable sampler

This was found while debugging venus-lavapipe ci failure. It's a real
bug though no tests have caught this yet, but fixing this would regress
venus-lavapipe non-templated push tests if without the dependent lvp
fix. The sampler in the descriptor write can be garbled if the binding
has immutable samplers.

cc: mesa-stable

Signed-off-by: Yiwei Zhang <zzyiwei@chromium.org>
Reviewed-by: Corentin Noël <corentin.noel@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20471>