drm/radeon: rework audio option

In 3.12 I changed audio to be enabled by default,
but you still had to turn it on via xrandr.  This
was confusing to users so change it to minic the
previous behavior:

- audio option is set to -1 (auto) by default which is
  the current 3.12 behavior (audio is enabled but requires
  xrandr to turn it on).
- if audio = 1, the audio is enabled without needing
  to mess with xrandr (previous behavior)
- audio = 0 disables audio

It retains the new feature of allowing the user to enable
audio on the fly with xrandr, but turns audio on
automatically if radeon.audio=1 is set which is what
most users expect.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>