drm/xe: Generalize wa bb emission code

drm/xe: Generalize wa bb emission code

Generalize the wa bb emission by splitting it into three phases - setup,
emit and finish, and extract setup and finish steps into helpers.

This will enable using the same infrastructure for emitting the indirect
context workarounds.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20250711160153.49833-2-tvrtko.ursulin@igalia.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>