drm/xe/configfs: Add mid context restore bb

Like done for post context restore, allow the user to add commands to
the middle of context restore, at the beginning of engine restore
commands.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250916-wa-bb-cmds-v5-7-306bddbc15da@intel.com


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