drm/xe/configfs: Extract function to parse engine

Move the part that copies the engine to a local buffer so it can be
shared in future for other configfs attributes parsing an engine.

Reviewed-by: Raag Jadav <raag.jadav@intel.com>
Link: https://lore.kernel.org/r/20250916-wa-bb-cmds-v5-1-306bddbc15da@intel.com


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