drm/i915: Update shared stats to use the new gem helper

Switch to using the new gem shared memory stats helper
rather than hand rolling it.

Link: https://lore.kernel.org/all/20231207180225.439482-1-alexander.deucher@amd.com/


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>