st/mesa: fix fragment shader output mapping

st/mesa: fix fragment shader output mapping

Properly handle the case where there is a gap in the assigned output locations,
e.g. a fragment shader writes to color buffer 2 but not to color buffers 0 & 1.

Fixes GL45-CTS.gtf33.GL3Tests.explicit_attrib_location.explicit_attrib_location_pipeline.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>