drm/i915/cml: Introduce Comet Lake PCH

Comet Lake PCH is based off of Cannon Point(CNP).
Add PCI ID for Comet Lake PCH.

v2: Code cleanup (DK)

v3: Comment cleanup (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190318200133.9666-2-anusha.srivatsa@intel.com