drm/xe/guc: Don't allocate temporary policies object

Since we are already using reusable buffer objects from the GuC
buffer cache, we can directly write into their CPU pointers and
spare unnecessary temporary allocation.

While around, also make sure to clear obtained buffer, to avoid
sending some stale data.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250702142504.1656-1-michal.wajdeczko@intel.com