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.

Fixes: baf61664 ("drm/connector: implement generic HDMI audio helpers")
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250527-hdmi-audio-helper-remove-fix-v1-1-6cf77de364d8@collabora.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>