drm/bridge: add function interface for DisplayPort audio implementation

It is common for the DisplayPort bridges to implement audio support. In
preparation to providing a generic framework for DP audio, add
corresponding interface to struct drm_bridge. As suggested by Maxime
for now this is mostly c&p of the corresponding HDMI audio API.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20250314-dp-hdmi-audio-v6-2-dbd228fa73d7@oss.qualcomm.com


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