turnip: Fix immutable samplers

Emma Anholt requested to merge anholt/mesa:tu-imm-samp into master

turnip: Fix support for immutable samplers.

We were setting up the hardware sampler state when updating a combined image sampler, but never looking at the immutable sampler for in the separate case.

Fixes failures in dEQP-VK.binding_model.shader_access.primary_cmd_buf.sampler_immutable.fragment.*

(NOTE: first two commits are from !3124 (merged), just the last commit is this MR.)

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