drm/i915: Respect alternate_aux_channel for all DDI ports

The VBT provides the platform a way to mix and match the DDI ports vs.
AUX channels. Currently we only trust the VBT for DDI E, which has no
corresponding AUX channel of its own. However it is possible that some
board might use some non-standard DDI vs. AUX port routing even for
the other ports. Perhaps for signal routing reasons or something,
So let's generalize this and trust the VBT for all ports.

For now we'll limit this to DDI platforms, as we trust the VBT a bit
more there anyway when it comes to the DDI ports. I've structured
the code in a way that would allow us to easily expand this to
other platforms as well, by simply filling in the ddi_port_info.

v2: Drop whitespace changes, keep MISSING_CASE() for unknown
    aux ch assignment, include a commit message, include debug
    message during init

Cc: stable@vger.kernel.org
Cc: Maarten Maathuis <madman2003@gmail.com>
Tested-by: Maarten Maathuis <madman2003@gmail.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=97877
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1476208368-5710-2-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>