drm/xe/configfs: Add post context restore bb

drm/xe/configfs: Add post context restore bb

Allow the user to specify commands to execute during a context restore.
Currently it's possible to parse 2 types of actions:

	- cmd: the instructions are added as is to the bb
	- reg: just use the address and value, without worrying about
	  encoding the right LRI instruction. This is possibly the most
	  useful use case, so added a dedicated action for that.

This also prepares for future BBs: mid context restore and rc6 context
restore that can re-use the same parsing functions.

Reviewed-by: Raag Jadav <raag.jadav@intel.com>
Link: https://lore.kernel.org/r/20250916-wa-bb-cmds-v5-4-306bddbc15da@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>