radeonsi: make SI_SGPR_VERTEX_BUFFERS the last user SGPR input

radeonsi: make SI_SGPR_VERTEX_BUFFERS the last user SGPR input

so that it can be removed and replaced with inline VBO descriptors,
and the pointer can be packed in unused bits of VBO descriptors.
This also removes the pointer from merged TES-GS where it's useless.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>