drm/i915: Protect older gen against intel_gt_init_powersave()

In the middle of intel_gt_init_powersave() we have an if-chain that ends
with a universal else clause to read gen6+ registers. Older platforms
like Pineview that end up here do not like those registers and may even
OOPS whilst reading them!

Fixes: 3ea9a80132 ("drm/i915: Perform static RPS frequency setup ...")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470132927-1821-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

drivers/gpu/drm/i915/intel_pm.c

@@ -6493,7 +6493,7 @@ void intel_init_gt_powersave(struct drm_i915_private *dev_priv)
 		cherryview_init_gt_powersave(dev_priv);
 	else if (IS_VALLEYVIEW(dev_priv))
 		valleyview_init_gt_powersave(dev_priv);
-	else
+	else if (INTEL_GEN(dev_priv) >= 6)
 		gen6_init_rps_frequencies(dev_priv);
 
 	/* Derive initial user preferences/limits from the hardware limits */