drm/radeon/kms: ATPX switcheroo fixes

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>