dma-fence: Make ->wait callback optional

dma-fence: Make ->wait callback optional

Almost everyone uses dma_fence_default_wait.

Reviewed-by: Christian König <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Cc: Sumit Semwal <[email protected]>
Cc: Gustavo Padovan <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]