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

Commit c5d822dad90 added support for sample mask incorrectly. It became
treated as a color output, and messed up the color output indices.
Revert the hunk that did that, and add explicit support just like for
depth/stencil writes.

Signed-off-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>
Acked-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>

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