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.)