drm/i915: Respect alternate_ddc_pin for all DDI ports

The VBT provides the platform a way to mix and match the DDI ports vs.
GMBUS pins. Currently we only trust the VBT for DDI E, which I suppose
has no standard GMBUS pin assignment. However, there are machines out
there that use a non-standard mapping for the other ports as well.
Let's start trusting the VBT on this one for all ports on DDI platforms.

I've structured the code such that other platforms could easily start
using this as well, by simply filling in the ddi_port_info. IIRC there
may be CHV system that might actually need this.

v2: Include a commit message, include a debug message during init

Cc: stable@vger.kernel.org
Cc: Maarten Maathuis <madman2003@gmail.com>
Tested-by: Maarten Maathuis <madman2003@gmail.com>
Bugzilla: 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/1476208368-5710-3-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
(cherry picked from commit e4ab73a1

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>