drm/i915: Remove DAC disable in CRT force detect on IGDNG

This is not required on newer stepping hardware to get
reliable force detect status. Removing this fixes screen
blank flicker in CRT detect on IGDNG.

Cc: Stable Team <stable@kernel.org>
Signed-off-by: NZhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>

drivers/gpu/drm/i915/intel_crt.c

@@ -179,13 +179,10 @@ static bool intel_igdng_crt_detect_hotplug(struct drm_connector *connector)
 {
 	struct drm_device *dev = connector->dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
-	u32 adpa, temp;
+	u32 adpa;
 	bool ret;
 
-	temp = adpa = I915_READ(PCH_ADPA);
-
-	adpa &= ~ADPA_DAC_ENABLE;
-	I915_WRITE(PCH_ADPA, adpa);
+	adpa = I915_READ(PCH_ADPA);
 
 	adpa &= ~ADPA_CRT_HOTPLUG_MASK;
 
@@ -212,8 +209,6 @@ static bool intel_igdng_crt_detect_hotplug(struct drm_connector *connector)
 	else
 		ret = false;
 
-	/* restore origin register */
-	I915_WRITE(PCH_ADPA, temp);
 	return ret;
 }