drm/i915/cnp: Introduce Cannonpoint PCH.

Most of south engine display that is in PCH is still the
same as SPT and KBP, except for this key differences:

- Backlight: Backlight programming changed in CNP PCH.
- Panel Power: Sligh programming changed in CNP PCH.
- GMBUS and GPIO: The pin mapping has changed in CNP PCH.

All of these changes follow more the BXT style.

v2: Update definition to use dev_priv isntead of dev (Tvrtko).

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.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/1496434004-29812-1-git-send-email-rodrigo.vivi@intel.com