drm/i915: implement WADPOClockGatingDisable for LPT

This should prevent mode set failures on LPT.
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Pimp the w/a tag to fit into Damien's new scheme.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -4093,6 +4093,11 @@ static void lpt_init_clock_gating(struct drm_device *dev)
 		I915_WRITE(SOUTH_DSPCLK_GATE_D,
 			   I915_READ(SOUTH_DSPCLK_GATE_D) |
 			   PCH_LP_PARTITION_LEVEL_DISABLE);
+
+	/* WADPOClockGatingDisable:hsw */
+	I915_WRITE(_TRANSA_CHICKEN1,
+		   I915_READ(_TRANSA_CHICKEN1) |
+		   TRANS_CHICKEN1_DP0UNIT_GC_DISABLE);
 }
 
 static void lpt_suspend_hw(struct drm_device *dev)