drm/i915: Add ECOBITS_SNB_BIT

GAC_ECO_BITS has a bit similar to GAM_ECOCHK's ECOCHK_SNB_BIT. Add
the define, and enable it on SNB.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>