translate_sse: Use the correct buffer index in this fast path.

translate_sse: Use the correct buffer index in this fast path.

It is possible that there are multiple input buffers but only one is
relevant for translation. Then there will be only a single translation
group, which might need to source data from a buffer index != 0.

Fixes wrong vertex shader inputs as observed while debugging with an
application and driver combination that requires translation of a
vertex attribute in a non-trivial set of attributes and input buffers.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>