drm/i915: Simplify hpd pin to port

We will soon need to make that pin port association per
platform, so let's try to simplify it beforehand.

Also we are moving the backwards port to pin
here as well so let's use a standardized way.

One extra possibility here would be to add a
MISSING_CASE along with PORT_NONE, but I don't want
to change this behaviour for now.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170811182650.14327-1-rodrigo.vivi@intel.com