drm/i915: Enable full PPGTT on gen7

Use full PPGTT as the default option in gen7.

Note that aliasing PPGTT is the default option for gen8 (see
HAS_PPGTT) since we're still fighting troubles around context
switching and execlists.

This may well come back to bite me later.
Signed-off-by: NMichel Thierry <michel.thierry@intel.com>
[danvet: Explain that gen8 full ppgtt is blocked on execlists for
now.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -59,7 +59,7 @@ static int sanitize_enable_ppgtt(struct drm_device *dev, int enable_ppgtt)
 		return 0;
 	}
 
-	return HAS_ALIASING_PPGTT(dev) ? 1 : 0;
+	return HAS_PPGTT(dev) ? 2 : HAS_ALIASING_PPGTT(dev) ? 1 : 0;
 }