dma-buf: add reservation_object_fences helper

Add a new helper to get a consistent set of pointers from the reservation
object. While at it group all access helpers together in the header file.

v2: correctly return shared_count as well

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/322378/?series=64837&rev=1