drm/radeon: add a connector property for audio

This provides a connector property to enable/disable hdmi
audio on the fly.  The default is disabled, but you can select
auto (let the driver detect an audio capable monitor and enable it)
or enabled (force audio enabled).  This also enables audio by
default so you no longer need a module parameter to enable audio.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>