drm/xe/configfs: Use guard() for dev->lock

Instead of the manual lock()/unlock() pattern, use guard() which will
make things easier for handling errors or early returns.

Reviewed-by: Dnyaneshwar Bhadane <dnyaneshwar.bhadane@intel.com>
Link: https://lore.kernel.org/r/20250821-psmi-v5-8-34ab7550d3d8@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>