drm/i915: Fix locking in DRRS flush/invalidate hooks

We must acquire the mutex before we can check drrs.dp, otherwise
someone might sneak in with a modeset, clear the pointer after we've
checked it and then the code will Oops.

This issue has been introduced in

commit a93fad0f
Author: Vandana Kannan <vandana.kannan@intel.com>
Date:   Sat Jan 10 02:25:59 2015 +0530

    drm/i915: DRRS calls based on frontbuffer

v2: Don't blow up on uninitialized mutex and work item by checking
whether DRRS is support or not first. Also unconditionally initialize
the mutex/work item to avoid future trouble.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Vandana Kannan <vandana.kannan@intel.com>
Cc: stable@vger.kernel.org (4.0+ only)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_dp.c

@@ -5157,7 +5157,6 @@ static void intel_edp_drrs_downclock_work(struct work_struct *work)
 			downclock_mode->vrefresh);
 
 unlock:
-
 	mutex_unlock(&dev_priv->drrs.mutex);
 }
 
@@ -5179,12 +5178,17 @@ void intel_edp_drrs_invalidate(struct drm_device *dev,
 	struct drm_crtc *crtc;
 	enum pipe pipe;
 
-	if (!dev_priv->drrs.dp)
+	if (dev_priv->drrs.type == DRRS_NOT_SUPPORTED)
 		return;
 
 	cancel_delayed_work_sync(&dev_priv->drrs.work);
 
 	mutex_lock(&dev_priv->drrs.mutex);
+	if (!dev_priv->drrs.dp) {
+		mutex_unlock(&dev_priv->drrs.mutex);
+		return;
+	}
+
 	crtc = dp_to_dig_port(dev_priv->drrs.dp)->base.base.crtc;
 	pipe = to_intel_crtc(crtc)->pipe;
 
@@ -5218,12 +5222,17 @@ void intel_edp_drrs_flush(struct drm_device *dev,
 	struct drm_crtc *crtc;
 	enum pipe pipe;
 
-	if (!dev_priv->drrs.dp)
+	if (dev_priv->drrs.type == DRRS_NOT_SUPPORTED)
 		return;
 
 	cancel_delayed_work_sync(&dev_priv->drrs.work);
 
 	mutex_lock(&dev_priv->drrs.mutex);
+	if (!dev_priv->drrs.dp) {
+		mutex_unlock(&dev_priv->drrs.mutex);
+		return;
+	}
+
 	crtc = dp_to_dig_port(dev_priv->drrs.dp)->base.base.crtc;
 	pipe = to_intel_crtc(crtc)->pipe;
 	dev_priv->drrs.busy_frontbuffer_bits &= ~frontbuffer_bits;
@@ -5294,6 +5303,9 @@ intel_dp_drrs_init(struct intel_connector *intel_connector,
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct drm_display_mode *downclock_mode = NULL;
 
+	INIT_DELAYED_WORK(&dev_priv->drrs.work, intel_edp_drrs_downclock_work);
+	mutex_init(&dev_priv->drrs.mutex);
+
 	if (INTEL_INFO(dev)->gen <= 6) {
 		DRM_DEBUG_KMS("DRRS supported for Gen7 and above\n");
 		return NULL;
@@ -5312,10 +5324,6 @@ intel_dp_drrs_init(struct intel_connector *intel_connector,
 		return NULL;
 	}
 
-	INIT_DELAYED_WORK(&dev_priv->drrs.work, intel_edp_drrs_downclock_work);
-
-	mutex_init(&dev_priv->drrs.mutex);
-
 	dev_priv->drrs.type = dev_priv->vbt.drrs_type;
 
 	dev_priv->drrs.refresh_rate_type = DRRS_HIGH_RR;