drm/amd/display: try to find matching audio inst for enc inst first

[Description]
in eDP+ HDMI/DP clone or extended configuration, audio inst changed from inst 1 to inst0.
No failure related this though, just playback device endpoint inst changed.
Also  remove one addition register read.

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>