apple-gmux: Add display mux support

Add support for the gmux display muxing functionality and register a mux
handler with vga_switcheroo.

Signed-off-by: Andreas Heider <andreas@meetr.de>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>