drm/i915/cnl: Introduce Cannonlake platform defition.

Cannonlake is a Intel® Processor containing Intel® HD Graphics
following Kabylake.

It is Gen10.

Let's start by adding the platform definition based on previous
platforms but yet as alpha_support.

On following patches we will start adding PCI IDs and the
platform specific changes.

CNL has an increased DDB size as Damien had previously
noticed and provided a separated patch that got squashed here.

v2: Squash DDB size here per Ander request.

Credits-to: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1496781040-20888-1-git-send-email-rodrigo.vivi@intel.com