drm/radeon/evergreen: set SAD registers

This allows audio (alsa) driver to read them and have a clue about audio
capabilities of connected receiver. This has been verified to be
compatible with fglrx behaviour for Onkyo TX-SR605 and Denon 1912.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>