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.