drm/xe/lrc: Prepare WA BB setup for more users

The post context restore (WA BB) is a mechanism in HW that may be used
for things other than the utilization setup. Create a new function
called setup_wa_bb() that wraps any function writing useful commands in
the buffer.

Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Link: https://lore.kernel.org/r/20250604-wa-bb-fix-v1-2-0dfc5dafcef0@intel.com


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