drm/i915/crt: Make sure the hotplug interrupt is enabled

After disabling the hotplug interrupts for VGA detection on Ironlake, be
sure to re-enable them again afterwards.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=30378
Signed-off-by: Yuanhan Liu <yuanhan.liu@intel.com>
Cc: stable@kernel.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_crt.c

@@ -191,7 +191,8 @@ static bool intel_ironlake_crt_detect_hotplug(struct drm_connector *connector)
 		DRM_DEBUG_KMS("timed out waiting for FORCE_TRIGGER");
 
 	if (turn_off_dac) {
-		I915_WRITE(PCH_ADPA, temp);
+		/* Make sure hotplug is enabled */
+		I915_WRITE(PCH_ADPA, temp | ADPA_CRT_HOTPLUG_ENABLE);
 		(void)I915_READ(PCH_ADPA);
 	}