drm/i915: fix debugging compilation error from previous commit

There is no equivalent to mutex_destroy() for spinlocks so just delete
the code.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_dma.c

@@ -2221,7 +2221,6 @@ void i915_driver_postclose(struct drm_device *dev, struct drm_file *file)
 {
 	struct drm_i915_file_private *file_priv = file->driver_priv;
 
-	mutex_destroy(&file_priv->mutex);
 	kfree(file_priv);
 }