dma-fence: Shrink size of struct dma_fence

Rearrange the couple of 32-bit atomics hidden amongst the field of
pointers that unnecessarily caused the compiler to insert some padding,
shrinks the size of the base struct dma_fence from 80 to 72 bytes on
x86-64.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190817144736.7826-3-chris@chris-wilson.co.uk