drm/i915: HSW: allow PCH clock gating for suspend

For the device to enter D3 we should enable PCH clock gating.

v2:
- use HAS_PCH_LPT instead of IS_HASWELL (Ville, Paolo)
- rename lpt_allow_clock_gating to lpt_suspend_hw (Paolo)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index 5093b86..22e3af6 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -3988,6 +3988,18 @@
 			   PCH_LP_PARTITION_LEVEL_DISABLE);
 }
 
+static void lpt_suspend_hw(struct drm_device *dev)
+{
+	struct drm_i915_private *dev_priv = dev->dev_private;
+
+	if (dev_priv->pch_id == INTEL_PCH_LPT_LP_DEVICE_ID_TYPE) {
+		uint32_t val = I915_READ(SOUTH_DSPCLK_GATE_D);
+
+		val &= ~PCH_LP_PARTITION_LEVEL_DISABLE;
+		I915_WRITE(SOUTH_DSPCLK_GATE_D, val);
+	}
+}
+
 static void haswell_init_clock_gating(struct drm_device *dev)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
@@ -4340,6 +4352,12 @@
 	dev_priv->display.init_clock_gating(dev);
 }
 
+void intel_suspend_hw(struct drm_device *dev)
+{
+	if (HAS_PCH_LPT(dev))
+		lpt_suspend_hw(dev);
+}
+
 /**
  * We should only use the power well if we explicitly asked the hardware to
  * enable it, so check if it's enabled and also check if we've requested it to