drm/i915/glk: Introduce Geminilake platform definition

Geminilake is an Intel® Processor containing Intel® HD Graphics
following Broxton.

Let's start by adding the platform definition. PCI IDs and plaform
specific code will follow.

v2: Rebase (don't allow dev to be used with the new macro).

v3: Update ddb size. (Matt)
    Rebase on s/preliminary_hw/alpha/

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479133526-32389-1-git-send-email-ander.conselvan.de.oliveira@intel.com