drm/i915: Add missing mutex_lock(&dev->struct_mutex)

there might be a nicer way to fix this but this is the simplest for now.
Signed-off-by: Pierre Willenbrock <pierre@pirsoft.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/i915/intel_display.c

@@ -1008,6 +1008,7 @@ static int intel_crtc_cursor_set(struct drm_crtc *crtc,
 		temp = CURSOR_MODE_DISABLE;
 		addr = 0;
 		bo = NULL;
+		mutex_lock(&dev->struct_mutex);
 		goto finish;
 	}