anv: Refactor pipeline l3 config setup

anv: Refactor pipeline l3 config setup

Now that we're using gen_l3_config.c, we no longer have one set of l3
config functions per gen and we can simplify a bit.  Also, we know that
only compute uses SLM so we don't need to look for it in all of the stages.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>