drm/radeon: Cleanup HDMI audio interrupt handling for evergreen

Same as the previous patch, but now for handling HDMI audio interrupts.

Changes since v1:
- Preserve the order we write back all registers

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Lyude <lyude@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>