drm/i915: More cargo-culted locking for intel_update_fbc

Just for consistency, this patch won't fix anything really.

v2: Rebase over all the recent plane enabling shuffling.

Reviewed-by: Akash Goel <akash.goel@intel.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -4618,7 +4618,9 @@ static void i9xx_crtc_disable(struct drm_crtc *crtc)
 	intel_crtc->active = false;
 	intel_update_watermarks(crtc);
 
+	mutex_lock(&dev->struct_mutex);
 	intel_update_fbc(dev);
+	mutex_unlock(&dev->struct_mutex);
 }
 
 static void i9xx_crtc_off(struct drm_crtc *crtc)