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

drivers/gpu/drm/i915/gt/intel_rc6.c

@@ -612,6 +612,9 @@ void intel_rc6_park(struct intel_rc6 *rc6)
 		return;
 	}
 
+	if (!(rc6->ctl_enable & GEN6_RC_CTL_RC6_ENABLE))
+		return;
+
 	/* Turn off the HW timers and go directly to rc6 */
 	set(uncore, GEN6_RC_CONTROL, GEN6_RC_CTL_RC6_ENABLE);
 	set(uncore, GEN6_RC_STATE, 0x4 << RC_SW_TARGET_STATE_SHIFT);