dma-buf: rework stub fence initialisation v2

dma-buf: rework stub fence initialisation v2

Instead of doing this on the first call of the function just initialize
the stub fence during kernel load.

This has the clear advantage of lower overhead and also doesn't rely on
the ops to not be NULL any more.

v2: use correct signal function

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Link: https://lore.kernel.org/r/20251031134442.113648-3-christian.koenig@amd.com