Improve Apple GMUX support on switcheroo

The Apple GMUX switcheroo code doesn't really work that well - there's no straightforward way to ensure that drivers are loaded in the correct order, and no mechanism to probe displays without performing a full switch. This patchset adds infrastructural support to switcheroo and functionality to GMUX in order to fix things up. This won't actually *work* in its current form - it needs additional patches to the GPU drivers, which are currently in a somewhat hacky state.