drm/i915: enable rc6 on ilk again

I have the faint hope that the total absence of any locking for the
rps code wasn't too good an idea and could very well have caused some
rc6 related regressions.

Unfortunately we've never managed to reproduce these issues on any of
our own machines, so the only way to go about this is to enable it and
see what happens.

While at it, kill some stale comments and improve the logging.
Acked-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -2367,31 +2367,26 @@ static void gen6_disable_rps(struct drm_device *dev)
 
 int intel_enable_rc6(const struct drm_device *dev)
 {
-	/*
-	 * Respect the kernel parameter if it is set
-	 */
+	/* Respect the kernel parameter if it is set */
 	if (i915_enable_rc6 >= 0)
 		return i915_enable_rc6;
 
-	/*
-	 * Disable RC6 on Ironlake
-	 */
-	if (INTEL_INFO(dev)->gen == 5)
-		return 0;
+	if (INTEL_INFO(dev)->gen == 5) {
+		DRM_DEBUG_DRIVER("Ironlake: only RC6 available\n");
+		return INTEL_RC6_ENABLE;
+	}
 
-	/* On Haswell, only RC6 is available. So let's enable it by default to
-	 * provide better testing and coverage since the beginning.
-	 */
-	if (IS_HASWELL(dev))
+	if (IS_HASWELL(dev)) {
+		DRM_DEBUG_DRIVER("Haswell: only RC6 available\n");
 		return INTEL_RC6_ENABLE;
+	}
 
-	/*
-	 * Disable rc6 on Sandybridge
-	 */
+	/* snb/ivb have more than one rc6 state. */
 	if (INTEL_INFO(dev)->gen == 6) {
 		DRM_DEBUG_DRIVER("Sandybridge: deep RC6 disabled\n");
 		return INTEL_RC6_ENABLE;
 	}
+
 	DRM_DEBUG_DRIVER("RC6 and deep RC6 enabled\n");
 	return (INTEL_RC6_ENABLE | INTEL_RC6p_ENABLE);
 }