drm/msm/dp: move/inline audio related functions

Move audio-related functions to dp_audio.c, following up the cleanup
done by the rest of the submodules. Inline functions with simple
register access patterns.

Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Tested-by: Stephen Boyd <swboyd@chromium.org> # sc7180-trogdor
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/654325/
Link: https://lore.kernel.org/r/20250518-fd-dp-audio-fixup-v6-7-2f0ec3ec000d@oss.qualcomm.com