st/nir/radeonsi: move nir_lower_uniforms_to_ubo() to the state tracker

st/nir/radeonsi: move nir_lower_uniforms_to_ubo() to the state tracker

This will only ever be used by gallium drivers so it probably doesn't
belong in the nir toolkit. Also we want to pass it some non NIR
things in the following patch.

To avoid regressions we wrap the lowering calls that have been moved
to st_glsl_to_nir with a quick hack so that they are only called for
radeonsi, we will replace the hack with a check for uniform packing
in a following patch.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>