drm/radeon/kms: track audio engine state, do not use not setup timer

This is needed to enable audio support on devices using polling. In case user
decides to disable audio (module parameter) we still will try to use timer in
r600_audio_enable_polling. This would lead to BUG in kernel/timer.c.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>