apple-gmux: Add switch_ddc support

Originally by Seth Forshee <seth.forshee@canonical.com>, 2012-10-04:
    The gmux allows muxing the DDC independently from the display, so
    support this functionality. This will allow reading the EDID for the
    inactive GPU, fixing issues with machines that either don't have a
    VBT or have invalid mode data in the VBT.

Modified by Lukas Wunner <lukas@wunner.de>, 2015-04 - 2015-12:
    Change semantics of ->switch_ddc handler callback to return previous
    DDC owner. Original version tried to determine previous DDC owner
    with find_active_client() in vga_switcheroo but this fails if the
    inactive client registers before the active client.

    v2.4: Retain semantics of ->switchto handler callback to switch all
          pins, including DDC (Daniel Vetter)

    v4:   Advertise ->switch_ddc handler callback only on the pre-retina
          Macbook Pro. The retina uses eDP instead of LVDS and gmux no
          longer does the muxing itself but merely controls an external
          mux. That mux is incapable of switching the AUX channel
          separately from the main link. It's an NXP CBTL06142
          (alternate parts: TI HD3SS212, Pericom PI3VDP12412,
          see datasheets below).

    v5:   Rebase on "apple-gmux: Track switch state".
          Rebase on "vga_switcheroo: Add handler flags infrastructure".
          Rebase on 5d170139eb10 ("Constify vga_switcheroo_handler"),
          requires 2 structs, 1x with ->switchto for pre-retinas,
          1x without for retinas).
          Add error message if handler registration with vga_switcheroo
          fails.

    Teardowns identifying the mux:
    http://www.electronicproducts.com/-whatsinside_text-145.aspx
    http://slideshare.net/jjwu6266/apple-2012-wwdc-apple-macbook-pro-with-retina-display
    http://www.techrepublic.com/blog/cracking-open/teardown-shows-retina-macbook-pro-is-nearly-impossible-to-upgrade-difficult-to-work-on/

    Mux Datasheets:
    http://www.nxp.com/documents/data_sheet/CBTL06141.pdf
    http://www.ti.com/lit/ds/symlink/hd3ss212.pdf
    https://www.pericom.com/assets/Datasheets/PI3VDP12412.pdf

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=88861
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=61115
Tested-by: Lukas Wunner <lukas@wunner.de>
    [MBP  9,1 2012  intel IVB + nvidia GK107  pre-retina  15"]
Cc: Seth Forshee <seth.forshee@canonical.com>
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/17fe8bfb0415d713bb4174f84ac9aae5d7d9a5f8.1452525860.git.lukas@wunner.de

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