drm/i915: Set GAC_ECO_BITS register on Gen7+

According to BSpec GAC_ECO_BITS register exists on Gen7 platforms as
well. Configure it accordingly.

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>