drm/i915/gt: Fix rc6 on Ivybridge

The current rc6 threshold is larger than the evaluation interval on
Ivybridge; it never enters rc6. Remove the special casing so it behaves
like the other gen6/gen7, and we see rc6 residencies before we manually
park the system.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/1114


Testcase: igt/i915_pm_rc6_residency/rc6-idle #ivb
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200203202110.670209-1-chris@chris-wilson.co.uk