drm/radeon/kms: disable hdmi audio by default

The current RE'd code causes blank screens and
display problems on a lot of systems.  So disable
it by default for now.  It can still be enabled
by setting the audio parameter to 1.  E.g.:
radeon.audio=1

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=38010
https://bugs.freedesktop.org/show_bug.cgi?id=27731
https://bugs.freedesktop.org/show_bug.cgi?id=35970
https://bugs.freedesktop.org/show_bug.cgi?id=26195


and many other reported problems.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>