drm/msm/dp: protect audio functionality during link maintenance

Link maintenance request from sink can be issued asynchronously
along with audio on/off functionalities. Link maintenance may
switch off and on the clocks needed for audio functionalities.
Protect the audio functionalities using lock to make sure the
required clocks are on.

CRs-Fixed: 2123895
Change-Id: I33e977b1e18feb205427a76171fd9b41fe463262
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>
[cohens@codeaurora.org: Corrected a spelling mistake in
        comment block reported by checkpatch]
Signed-off-by: Steve Cohen <cohens@codeaurora.org>