drm/i915: Introduce GEN6_FEATURES for device info

Introducing a GEN6_FEAUTRES macro to simplify the struct definitions by
platforms given that most of the features are common. Inspired by the
GEN7_FEATURES macro done by Ben W. and others.

Use it for snb.

CC: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Carlos Santa <carlos.santa@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>