drm/amdkfd: Fix eviction fence handling

drm/amdkfd: Fix eviction fence handling

Handle case that dma_fence_get_rcu_safe returns NULL.

If restore work is already scheduled, only update its timer. The same
work item cannot be queued twice, so undo the extra queue eviction.

Fixes: 9a1c1339abf9 ("drm/amdkfd: Run restore_workers on freezable WQs")
Signed-off-by: Felix Kuehling <[email protected]>
Reviewed-by: Philip Yang <[email protected]>
Tested-by: Gang BA <[email protected]>
Reviewed-by: Gang BA <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>
Cc: [email protected]