radeonsi: link ES-GS just like LS-HS

radeonsi: link ES-GS just like LS-HS

This reduces the shader key for ES.

Use a fixed attrib location based on (semantic name,  index).

The ESGS item size is determined by the physical index of the highest ES
output, so it's almost always larger than before, but I think that
shouldn't matter as long as the ESGS ring buffer is large enough.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>