vga_switcheroo: Add a helper function to get the client state

Add vga_switcheroo_get_client_state() to get the current state of the
client.  This is necessary to determine the proper initial state of
audio clients in HD-audio driver.

Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>