drm/vc4: fix infinite EPROBE_DEFER loop

drm/vc4: fix infinite EPROBE_DEFER loop

`vc4_hdmi_audio_init` calls `devm_snd_dmaengine_pcm_register` which may
return EPROBE_DEFER. Calling `drm_connector_hdmi_audio_init` adds a
child device. The driver model docs[1] state that adding a child device
prior to returning EPROBE_DEFER may result in an infinite loop.

[1] https://www.kernel.org/doc/html/v6.14/driver-api/driver-model/driver.html

Fixes: 9640f1437a88 ("drm/vc4: hdmi: switch to using generic HDMI Codec infrastructure")
Signed-off-by: Gabriel Dalimonte <gabriel.dalimonte@gmail.com>
Link: https://lore.kernel.org/r/20250601-vc4-audio-inf-probe-v2-1-9ad43c7b6147@gmail.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>