drm/i915/bdw: IS_GEN8 definition

No PCI ids yet, so nothing should happen.

Rebase-Note: This one needs replacement ;-)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>