zink: consolidate/optimize descriptor struct filling

Working on a new contributor task marked by @zmike in #5377

Before I hack away much more, I thought I'd open the MR to see if this direction is acceptable to continue in. I also ran the Piglit suites sanity and llvmpipe_gl (with zink and lavapipe) to spot check that I didn't introduce a regression.

My next step would be to delete the following functions altogether, and update their callers.

update_descriptor_state_ubo
update_descriptor_state_ssbo
update_descriptor_state_sampler
update_descriptor_state_image

Along the way I'll see other ways to refactor this to keep the cleanup going. I feel this file's ripe for it.