drm/i915/icl: Add initial Icelake definitions.

Icelake is an Intel® Processor containing an Intel® Graphics
Controller.

This is just an initial Icelake definition. PCI IDs, Icelake support
and new features coming in following patches.

v2: Add .ddb_size and .has_guc (Michal Wajdeczko).
v3: Add the ICL_FEATURES macro (Kelvin Gardiner).
v4 (from Paulo): Add missing __initconst (Paulo) and say "graphics
controller" instead of something that looks like an official marketing
name but isn't (Chris).

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180111180010.24357-3-paulo.r.zanoni@intel.com