radeonsi: do not export VS outputs from vertex streams != 0

radeonsi: do not export VS outputs from vertex streams != 0

This affects for GS copy shaders. When an output is meant for vertex
stream != 0, then we don't have to make it available to the pixel
shader.

There is a minor inefficiency here because the GLSL varying packing pass
does not group varyings of the same vertex stream together, but it
shouldn't be important in practice.

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