Currently audio is not supported when DP is used for connecting the monitor.
This works if the monitor is connected using HDMI, or when fglrx is used.
I tried patching the driver with hardcoded values taken from the fglrx drivers
using radeontool, unfortunately, this was not sufficient.
1. added call in atombios_encoders.c:radeon_atom_encoder_mode_set(...) to
initialize audio when encoder mode is DP.
2. patched evergreen_hdmi.c:evergreen_hdmi_enable(...) and
evergreen_hdmi_setmode(...) to set same register values as fglrx.
3. patched dce6_afmt.c to set values matching fglrx.
Something is missing here ...