drm/xe/sa: Drop redundant NULL assignments

The sa_manager is drmm_kzalloc'ed so all members are already zero.
And in case of kvzalloc() failure we are not returning pointer to
the sa_manager at all, so no point in resetting .bo member.

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://patchwork.freedesktop.org/patch/msgid/20241220194205.995-3-michal.wajdeczko@intel.com