drm/xe: Expose user fence from xe_sync_entry

By allowing getting reference to user fence, we can
control the lifetime outside of sync entries.

This is needed to allow vma to track the associated
user fence that was provided with bind ioctl.

v2: xe_user_fence can be kept opaque (Jani, Matt)
v3: indent fix (Matt)

Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240215181152.450082-2-mika.kuoppala@linux.intel.com