drm/i915: Introduce GEN7_FEATURES for device info

Recommended by Chris.

v2: Make it GEN7_FEATURES, and use it for vlv and hsw also (Ben)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>