drm/amd/display: audio endpoint cannot switch

[Why]
On some systems, we need to check the dcn version in runtime
system, not in compile time.

[How]
Stub in dcn version parameter to find_first_free_audio

Signed-off-by: Paul Hsieh <paul.hsieh@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>