drm/mediatek: dp: Avoid mutex locks if audio is not supported/enabled

drm/mediatek: dp: Avoid mutex locks if audio is not supported/enabled

If a controller (usually, eDP!) does not support audio, or audio is not
enabled because the endpoint has no audio support, it's useless to lock
a mutex only to unlock it right after because there's no .plugged_cb().

Check if the audio is supported and enabled before locking the mutex in
mtk_dp_update_plugged_status(): if not, we simply return immediately.

While at it, since the update_plugged_status_lock mutex would not be
used if the controller doesn't support audio at all, initialize it
only if `audio_supported` is true.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20230725073234.55892-8-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>