dma-buf/fence-array: add fence_is_array()

Add helper to check if fence is array.

v2: Comments from Chris Wilson
	- remove ternary if from ops comparison
	- add EXPORT_SYMBOL(fence_array_ops)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>