drm/i915: Always define GEN as part of GENx_FEATURES

Be consistent and define the device's GEN as part of the GENx_FEATURE.
It will be overridden by the next gen upon inheriting, as per usual.

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/20180215081930.11477-2-chris@chris-wilson.co.uk