vga_switcheroo: Introduce struct vga_switcheroo_client_ops

This changes the API as a clean-up.  Instead of passing multiple
function pointers at each time, introduce a new struct holding the
whole callback functions and pass it to the registration.

The same struct will be used for the upcoming audio client
registration, too.

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