dma-buf: Split out dma fence array create into alloc and arm functions

Useful to preallocate dma fence array and then arm in path of reclaim or
a dma fence.

v2:
 - s/arm/init (Christian)
 - Drop !array warn (Christian)
v3:
 - Fix kernel doc typos (dim)

Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240826170144.2492062-2-matthew.brost@intel.com