drm/i915: Manipulate the Global GTT size using I915_GTT_PAGE_SIZE

I incorrectly converted the exclusion of the last 4096 bytes (that avoids
any potential prefetching past the end of the GTT) to PAGE_SIZE and not
to I915_GTT_PAGE_SIZE as it should be.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170206084547.27921-1-chris@chris-wilson.co.ukReviewed-by: Matthew Auld <matthew.auld@intel.com>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -3243,9 +3243,9 @@ int i915_ggtt_init_hw(struct drm_i915_private *dev_priv)
 	 * shrink the range used by drm_mm.
 	 */
 	mutex_lock(&dev_priv->drm.struct_mutex);
-	ggtt->base.total -= PAGE_SIZE;
+	ggtt->base.total -= I915_GTT_PAGE_SIZE;
 	i915_address_space_init(&ggtt->base, dev_priv, "[global]");
-	ggtt->base.total += PAGE_SIZE;
+	ggtt->base.total += I915_GTT_PAGE_SIZE;
 	if (!HAS_LLC(dev_priv))
 		ggtt->base.mm.color_adjust = i915_gtt_color_adjust;
 	mutex_unlock(&dev_priv->drm.struct_mutex);