drm/i915/icl: Apply a recommended rc6 threshold

On gen11 the recommended rc6 threshold differs from previous
gens, apply it. Move the write to a correct spot in sequence.

v2: do write in 2b, fix bspec ref (Michal)

Bspec: 33149
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190410105923.18546-2-mika.kuoppala@linux.intel.com