drm/i915: Drop mutex around clearing error state

The error state itself is guarded by a spinlock (admittedly even that is
overkill for a single pointer!) and doesn't require us to take the
struct_mutex in the debugfs/sysfs interface. Removing the struct_mutex
removes one more potential blockage when trying to debug a deadlock.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20160901205510.31307-1-chris@chris-wilson.co.uk
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com

drivers/gpu/drm/i915/i915_sysfs.c

@@ -556,17 +556,9 @@ static ssize_t error_state_write(struct file *file, struct kobject *kobj,
 {
 	struct device *kdev = kobj_to_dev(kobj);
 	struct drm_i915_private *dev_priv = kdev_minor_to_i915(kdev);
-	struct drm_device *dev = &dev_priv->drm;
-	int ret;
 
 	DRM_DEBUG_DRIVER("Resetting error state\n");
-
-	ret = mutex_lock_interruptible(&dev->struct_mutex);
-	if (ret)
-		return ret;
-
-	i915_destroy_error_state(dev);
-	mutex_unlock(&dev->struct_mutex);
+	i915_destroy_error_state(&dev_priv->drm);
 
 	return count;
 }