gallium/ureg: Sort the output decls.

glsl_to_tgsi had been emitting outputs in sorted order, and it seems virgl
has a dependency on that -- it collects outputs into an array and indexes
by the subtracting the ".First" value of the first one seen, even if
that's not the first entry in the array.

Fixes the regressions in deqp-gles2 on virgl with ST_DEBUG=force_nir.