vga_switcheroo: Enable/disable audio clients at the right time

The audio clients have to be disabled before disabling the VGA and
switching.  Similarly, enabling the audio client should be done at
last.  Otherwise the audio-side operation stalls, eventually leading
to Oops or lockups.

Tested-by: Jörg-Volker Peetz <jvpeetz@web.de>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>