dma-buf: add WARN_ON() illegal dma-fence signaling

Calling the signaling a NULL fence is obviously a coding error in a
driver. Those functions unfortunately just returned silently without
raising a warning.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20240911085903.1496-2-christian.koenig@amd.com