dma-buf: add dma_fence_was_initialized function v2

dma-buf: add dma_fence_was_initialized function v2

Some driver use fence->ops to test if a fence was initialized or not.
The problem is that this utilizes internal behavior of the dma_fence
implementation.

So better abstract that into a function.

v2: use a flag instead of testing fence->ops, rename the function, move
    to the beginning of the patch set.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Link: https://lore.kernel.org/r/20260120105655.7134-2-christian.koenig@amd.com