[Mesa-stable] Cherry-pick commits for 10.2

[Ilia Mirkin]

I totally spaced out on adding the CC stable on these:

mesa/st: fix color outputs in presence of sample mask output
ab7bd7093dfd18778ece4ed9098666c9ebc68d51

nvc0: revert mistaken logic to collapse color outputs to the beginning
0d699530ff9fe68a1898f875b889422536bfc8e5

They fix a discrepancy with color output indices when there is a
sample mask output in tgsi, which is included in 10.2 (only supported
by nouveau). In fairness, the sample mask tests still don't work even
with this fix, but I'm beginning to suspect it's due to unrelated
issues.

Thanks,

  -ilia