turnip: Fix support for immutable samplers.

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.*

Reviewed-by: Jonathan Marek <jonathan@marek.ca>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3127>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3127>