apple-gmux: Add suspend/resume support for the backlight

After S3, the brightness might not be restored to the pre-suspend value.
Request status update calls from the backlight core on suspend/resume to
ensure the brightness value is restored.

Reported-and-tested-by: Austin Lund <austin.lund@gmail.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>