anv: Drop has_slm in emit_l3_config for gen11+

anv: Drop has_slm in emit_l3_config for gen11+

For some gen12+ platforms, L3 config (cfg) can be NULL leading to a
seg-fault in emit_l3_config. But, we don't use has_slm for gen11+, so
we can just avoid declaring the variable.

Reworks:
 * Drop has_slm variable for all gens (suggested-by Jason)

Ref: 633dec7163e ("anv: Set L3 full way allocation at context init if L3 cfg is NULL")
Ref: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9534
Fixes: 581e68bc99b ("anv: move L3 config emission to genX_state.c")
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9589>