drm/msm/dp: fix audio during link maintenance

During link maintenance, audio needs to be switched off and
on to adapt to new link configurations. Move the registration
of audio functionalities with external-display module to
audio sub-module's initialization. This is needed to separate
out the audio initialization and registration from audio enable
and disable. Now, audio initialization and registration happens
during DisplayPort initialization. For MST case, it happens during
connector installation for a new stream.
This gives the flexibility to independently enabled or disabled
audio. It can be switched along with the video stream during
hot plug or during link maintenance.

CRs-Fixed: 2305680
Change-Id: I22a056af75a9851056c97e8174319c0dc3022f29
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>