Documentation/gpu: Fix section in the wrong scope

That section should still be inside "DRM client usage stats" rather than
as a sibling.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221107173209.2219571-2-lucas.demarchi@intel.com