drm/xe: Split xe_device_td_flush()

xe_device_td_flush() has 2 possible implementations: an entire L2 flush
or a transient flush, depending on WA 16023588340. Make this clear by
splitting the function so it calls each of them.

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://lore.kernel.org/r/20250618-wa-22019338487-v5-3-b888388477f2@intel.com


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