nvc0: revert mistaken logic to collapse color outputs to the beginning

In commit af38ef907, I added a "fix" to color outputs not being assigned
correctly when sample mask was being output. This was totally wrong --
the color indices (i.e. "si" values) were the ones that were wrong. Undo
that hunk.

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