drm/xe: Rework the preempt fence interface somewhat

Returning an embedded struct list_head when allocating a preempt fence leads to less code but will probably create confusion moving forward.

Fix that by returning a struct xe_preempt_fence instead and provide functions to conver to and from the list head.

No functional change. Prefixed with Draft pending more thorough testing.

Signed-off-by: Thomas Hellström thomas.hellstrom@linux.intel.com