drm/i915/skl: Restrict the ring frequency table programming to SKL

Ring frequency table programming is not required on BXT. Added separate
checks to enable the programming only for SKL & skip for BXT.

v2: Removed the BXT check from gen6_update_ring_freq function

Issue: VIZ-5144
Signed-off-by: NAkash Goel <akash.goel@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -6219,7 +6219,8 @@ static void intel_gen6_powersave_work(struct work_struct *work)
 	} else if (INTEL_INFO(dev)->gen >= 9) {
 		gen9_enable_rc6(dev);
 		gen9_enable_rps(dev);
-		__gen6_update_ring_freq(dev);
+		if (IS_SKYLAKE(dev))
+			__gen6_update_ring_freq(dev);
 	} else if (IS_BROADWELL(dev)) {
 		gen8_enable_rps(dev);
 		__gen6_update_ring_freq(dev);