ntb_perf: avoid false dma unmap of destination address

The DMA map and unmap of destination address is already being
done in perf_init_test() and perf_clear_test() functions.
Hence avoiding it by making necessary changes in perf_copy_chunk()
function.

Signed-off-by: Sanjay R Mehta <sanju.mehta@amd.com>
Signed-off-by: Arindam Nath <arindam.nath@amd.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>