drm/panthor: Fix the "done_fence is initialized" detection logic

drm/panthor: Fix the "done_fence is initialized" detection logic

After commit 541c8f2468b9 ("dma-buf: detach fence ops on signal v3"),
dma_fence::ops == NULL can't be used to check if the fence is initialized.
Use dma_fence_was_initialized() instead.

v2:
- Use dma_fence_was_initialized() instead of open-coding it

Cc: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Philipp Stanner <phasta@kernel.org>
Cc: Christian König <christian.koenig@amd.com>
Reported-by: Steven Price <steven.price@arm.com>
Reported-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Fixes: 541c8f2468b9 ("dma-buf: detach fence ops on signal v3")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Tested-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20260309124318.222902-1-boris.brezillon@collabora.com