apple-gmux: Add helper for presence detect

Centralize gmux' ACPI HID in a header file and add apple_gmux_present().
This can be used by other drivers to activate quirks specific to dual
GPU MacBook Pros & Mac Pros. The alternative would be to hardcode DMI
or PCI IDs and amend them whenever Apple introduces a new machine.

Tested-by: Lukas Wunner <lukas@wunner.de>
    [MBP  9,1 2012  intel IVB + nvidia GK107  pre-retina  15"]
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/89c23769058a340e5e11d4a7102f3793d3b0c94c.1452525860.git.lukas@wunner.de