intel/genxml: Drop SLMEnable from L3CNTLREG on Gen11

intel/genxml: Drop SLMEnable from L3CNTLREG on Gen11

SML is no longer in the L3$ on Gen11+.  It's not incredibly clear from
the docs but no Gen11 platforms are in the list of platforms on which
this bit exists.  Also, we've been always setting it false on Gen11 in
ANV and i965 thanks to GEN_L3P_SLM being zero with no ill effects.

Cc: "20.0" mesa-stable@lists.freedesktop.org
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3454>