drm/i915/gt: Use soft-rc6 for w/a protection

Now that we have soft-rc6 in place, we can use that instead of the
forcewake to disable rc6 while active; preferred by a few
microbenchmarks.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191202110836.2342685-1-chris@chris-wilson.co.uk