Improve Apple GMUX support on switcheroo

[Matthew Garrett]

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.

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Matthew Garrett | matthew.garrett at nebula.com