dma-buf: some dma_fence_chain improvements

The callback and the irq work are never used at the same
time. Putting them into an union saves us 24 bytes and
makes the structure only 120 bytes in size.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210611120301.10595-2-christian.koenig@amd.com