drm/i915/cnp: add CNP gmbus support

On CNP PCH based platforms the gmbus is on the south display that
is on PCH. The existing implementation for previous platforms
already covers the need for CNP expect for the pin pair configuration
that follows similar definitions that we had on BXT.

v2: Don't drop "_BXT" as the indicator of the first platform
    supporting this pin numbers. Suggested by Daniel.
v3: Add missing else and fix register table since CNP GPIO_CTL
    starts on 0xC5014.
v4: Fix pin number and map according to the current available VBT.
    Re-add pin 4 for port D. Lost during some rebase.
v5: Use table as spec. If VBT is wrong it should be ignored.

Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1496434004-29812-5-git-send-email-rodrigo.vivi@intel.com