drm/amd/display: Fixed the audio noise during mode switching with HDCP mode on

[Why]
When HDCP is on, some display would introduce audio noise during
HDCP handling.

[How]
Mute before HDCP handling when disabling core link. Unmute after
HDCP when enabling core link.

Signed-off-by: Roy Chan <roy.chan@amd.com>
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>