drm/amd/display: Expose audio inst from DC to DM

[Why]
In order to give pin notifications to the sound driver from DM we need
to know whether audio is enabled on a stream and what pin it's using
from DC.

[How]
Expose the instance via stream status if it's a mapped resource for
the stream. It will be -1 if there's no audio mapped.

Cc: Leo Li <sunpeng.li@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>