IB/mlx5: Ensure proper synchronization accessing memory

Call mlx5_ib_populate_pas() before mapping the DMA buffer to ensure
the hardware reads the values written by the CPU.

Found by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>