drm/i915/gtt: Switch gen8_free_page_tables params

After Mika's ppgtt cleanup series, all the other free functions have
drm_device as the first parameter, except this one.

No functional changes.

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>