drm/connector: only call HDMI audio helper plugged cb if non-null

In the Linux kernel, the following vulnerability has been resolved: drm/connector: only call HDMI audio helper plugged cb if non-null On driver remove, sound/soc/codecs/hdmi-codec.c calls the plugged_cb with NULL as the callback function and codec_dev, as seen in its hdmi_remove function. The HDMI audio helper then happily tries calling said null function pointer, and produces an Oops as a result. Fix this by only executing the callback if fn is non-null. This means the .plugged_cb and .plugged_cb_dev members still get appropriately cleared.