drm/i915/tgl: Implement Wa_1406941453

Enable Small PL for power benefit.

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190713010940.17711-18-lucas.demarchi@intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20190726000226.26914-4-lucas.demarchi@intel.com