dma-buf: Don't misuse dma_fence_signal()

dma-buf: Don't misuse dma_fence_signal()

The return code of dma_fence_signal() is not really useful as there is
nothing reasonable to do if a fence was already signaled. That return
code shall be removed from the kernel.

Moreover, dma_fence_signal() should not be used to check whether fences
are signaled. That's what dma_fence_is_signaled() and
dma_fence_test_signaled_flag() exist for.

Replace the non-canonical usage of dma_fence_signal().

Suggested-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patch.msgid.link/20251201105011.19386-7-phasta@kernel.org