drm/i915: Configure GAM_ECOCHK appropriatly for Gen7

IVB and HSW use different encodings for the PPGTT cacheability bits in
the GAM_ECOCHK register.

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>