drm/i915: Store gen_mask inside the static device info

Rather than deriving the gen_mask from the static intel_device_info->gen
at runtime, pre-fill it in the static data.

v2: Undefine local macros at end of their scope.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180215081930.11477-1-chris@chris-wilson.co.uk