nir/lower_tex: use a callback to check sampler return size packing

nir/lower_tex: use a callback to check sampler return size packing

The lower_tex_packing pass relies on the sampler index to access packing
information, but this is only valid for tex instructions that have sampler
state (so not txf, etc). Instead, let backends provide a callback to inform
the lowering about the packing used with a given texture instruction which
is more flexible.

Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24537>