When we switch the display mux, also switch
the i2c mux.  Also use the start and finish
methods to let the sbios know that the switch
is happening.

Should fix:
https://bugs.freedesktop.org/show_bug.cgi?id=35398



Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>