mesa: rework _mesa_add_parameter() to only add a single param

mesa: rework _mesa_add_parameter() to only add a single param

This is more inline with what the functions name suggests it should
do, and makes the code much easier to follow.

This will also make adding uniform packing support much simpler.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>