drm/i915: Use dedicated rc6 enabling sequence for gen11

In order not to inflate gen9 rc6 enabling sequence with
gen11 specifics, use a separate function for it.

Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190410105923.18546-1-mika.kuoppala@linux.intel.com