vga_switcheroo: Add the support for audio clients

Add the support for audio clients to VGA-switcheroo for handling the
HDMI audio controller together with VGA switching.  The id of the
audio controller should be given explicitly at registration time
unlike the video controller.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=43155



Signed-off-by: Takashi Iwai <tiwai@suse.de>