drm/i915: Conditionally carve out GGTT PDE

It only works that way on GEN6 and GEN7. Let's not assume GENn will be
the same.
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -623,9 +623,12 @@ void i915_gem_init_global_gtt(struct drm_device *dev)
 
 	if (intel_enable_ppgtt(dev) && HAS_ALIASING_PPGTT(dev)) {
 		int ret;
+
+		if (INTEL_INFO(dev)->gen <= 7) {
 			/* PPGTT pdes are stolen from global gtt ptes, so shrink the
 			 * aperture accordingly when using aliasing ppgtt. */
 			gtt_size -= I915_PPGTT_PD_ENTRIES*PAGE_SIZE;
+		}
 
 		i915_gem_setup_global_gtt(dev, 0, mappable_size, gtt_size);