drm/i915/gt: Only ignore rc6 parking for PCU on byt/bsw

An oversight in that we use rc6->ctl_enable to disable rc6 on gen9 and
so it does not simply indicate indirect control via a PCU. Switch the
rc6->ctl_enable check for a platform-based check.

Fixes: 972745fd

 ("drm/i915/gt: Disable manual rc6 for Braswell/Baytrail")
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/20191212072737.884335-2-chris@chris-wilson.co.uk