drm/i915/gt: Disable manual rc6 for Braswell/Baytrail

The initial investigated showed that while the PCU on Braswell/Baytrail
controlled RC6 itself. setting the software RC6 request made no
difference. Further testing reveals though that it causes a delay in the
PCU on enabling RC6.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/763


Fixes: 730eaeb5 ("drm/i915/gt: Manual rc6 entry upon parking")
Testcase: igt/perf/rc6-disable
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210180111.3958558-1-chris@chris-wilson.co.uk