drm/xe: Drop HW fence pointer to HW fence ctx

The HW fence ctx objects are not ref counted rather tied to the life of
an LRC object. HW fences reference the HW fence ctx, HW fences can
outlive LRCs thus resulting in UAF. Drop the  HW fence pointer to HW
fence ctx rather just store what is needed directly in HW fence.

v2:
 - Fix typo in commit (Ashutosh)
 - Use snprintf (Ashutosh)

Fixes: dd08ebf6 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240815193522.16008-1-matthew.brost@intel.com