drm/i915: Don't disable PPGTT for CHV based in PCI rev

In
 commit 62942ed7
 Author: Jesse Barnes <jbarnes@virtuousgeek.org>
 Date:   Fri Jun 13 09:28:33 2014 -0700

    drm/i915/vlv: disable PPGTT on early revs v3

we forgot about CHV. IS_VALLEYVIEW() is true for CHV, so we need to
explicitly avoid disabling PPGTT on CHV.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>