gpudev: use CPU mapping in communication list

gpudev: use CPU mapping in communication list

rte_gpu_mem_cpu_map() exposes a GPU memory area to the CPU.
In gpudev communication list this is useful to store the
status flag.

A communication list status flag allocated on GPU memory
and mapped for CPU visibility can be updated by CPU and polled
by a GPU workload.

The polling operation is more frequent than the CPU update operation.
Having the status flag in GPU memory reduces the GPU workload polling
latency.

If CPU mapping feature is not enabled, status flag resides in
CPU memory registered so it's visible from the GPU.

To facilitate the interaction with the status flag, this patch
provides also the set/get functions for it.

Signed-off-by: Elena Agostini <eagostini@nvidia.com>