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.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>