drm/i915: Introduce intel_hpd_pin function.

The idea is to have an unique place to decide the pin-port
per platform.

So let's create this function now without any functional
change. Just adding together code from hdmi and dp together.

v2: Add missing pin for port A.
v3: Fix typo on subject.
    Avoid behaviour change so add WARN_ON and return
    if port A on HDMI. (by DK).

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
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/20170811182650.14327-2-rodrigo.vivi@intel.com