drm/i915: cleanup rc6 code

Cleanup several aspects of the rc6 code:
  - misnamed intel_disable_clock_gating function (was only about rc6)
  - remove commented call to intel_disable_clock_gating
  - rc6 enabling code belongs in its own function (allows us to move the
    actual clock gating enable call back into restore_state)
  - allocate power & render contexts up front, only free on unload
    (avoids ugly lazy init at rc6 enable time)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[ickle: checkpatch cleanup]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_drv.c

@@ -355,10 +355,10 @@ static int i915_drm_thaw(struct drm_device *dev)
 
 		/* Resume the modeset for every activated CRTC */
 		drm_helper_resume_force_mode(dev);
-	}
 
-	/* Clock gating state */
-	intel_enable_clock_gating(dev);
+		if (dev_priv->renderctx && dev_priv->pwrctx)
+			ironlake_enable_rc6(dev);
+	}
 
 	intel_opregion_init(dev);