drm/xe/lrc: Allow to add user commands on context switch

During validation it's useful to allows additional commands to be
executed on context switch. Fetch the commands from configfs (to be
added) and add them to the WA BB.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250916-wa-bb-cmds-v5-3-306bddbc15da@intel.com


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