drm/i915: Clean up DDI DDC/AUX CH sanitation

Now that we use the AUX and GMBUS assignment from VBT for all ports,
let's clean up the sanitization of the port information a bit.
Previosuly we only did this for port E, and only complained about a
non-standard assignment for the other ports. But as we know that
non-standard assignments are a fact of life, let's expand the
sanitization to all the ports.

v2: Include a commit message, fix up the comments a bit
v3: Don't clobber other ports if the current port has no alternate aux ch/ddc pin

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/1476208368-5710-4-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jim Bride <jim.bride@linux.intel.com> (v2)