drm/amd/display: use encoder's engine id to find matched free audio device

[Why]
On some platforms, the encoder id 3 is not populated. So the encoders
are not stored in right order as index (id: 0, 1, 2, 4, 5) at pool. This
would cause encoders id 4 & id 5 to fail when finding corresponding
audio device, defaulting to the first available audio device. As result,
we cannot stream audio into two DP ports with encoders id 4 & id 5.

[How]
It need to create enough audio device objects (0 - 5) to perform matching.
Then use encoder engine id to find matched audio device.

Signed-off-by: Tai Man <taiman.wong@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>