drm/radeon/radeon_audio: Remove unused r600_hdmi_audio_workaround

The last use of r600_hdmi_audio_workaround() was removed by 2014's
commit 6e72376d ("radeon/audio: consolidate audio_mode_set()
functions")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>