drm/i915: Split pin mapping into per platform functions

Cleanup the code. Map the pins in accordance to
individual platforms rather than according to ports.
Create separate functions for platforms.

v2:
 - Add missing condition for CoffeeLake. Make platform
 specific functions static. Add function
 i915_ddc_pin_mapping().

v3:
 - Rename functions to x_port_to_ddc_pin() which directly
   indicates the purpose. Correct default return values on CNP
   and BXT. Rename i915_port_to_ to g4x_port_to since that was
   the first platform to run this. Correct code style. (Paulo)

Sugested-by Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Clinton Taylor <clinton.a.taylor@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1502927114-24012-1-git-send-email-anusha.srivatsa@intel.com