drm/msm/dp: fix mst audio register/deregister issue

The existing implementation registers with msm-ext-disp
module at dp_audio_get function with dp_panel stream-id.
The stream-id is invalid at this point as stream-id assignment
happens during the bridge_pre_enable. This change moves the
register/deregister calls into audio_on/audio_off functions
at the point where dp_panel holds the valid stream-id.

CRs-Fixed: 2325207
Change-Id: I415debb1e0431ff60729282d6afb41fa8fa6f1bf
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>