drm/i915: Track page table backing store usage

Account page table backing store against the owning client memory usage
stats.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Aravind Iddamsetty <aravind.iddamsetty@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231107101806.608990-3-tvrtko.ursulin@linux.intel.com