agx: Lower txs to a descriptor crawl

agx: Lower txs to a descriptor crawl

There's no native txs instruction... but we can emulate one :-) This is
heavy on shader ALU, but in the production driver, it'll all be hoisted
up to the preamble shader and so it shouldn't matter much. This
keeps the driver itself simple and low overhead, with a completely
obvious generalization to bindless.

Passes dEQP-GLES3.functional.shaders.texture_functions.texturesize.*

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18525>