IB/mlx5: Remove per-MR pas and dma pointers

Since UMR code now uses its own context struct on the stack, the pas
and dma pointers for the UMR operation that remained in the mlx5_ib_mr
struct are not necessary.  This patch removes them.

Fixes: a74d2416

 ("IB/mlx5: Refactor UMR to have its own context struct")
Signed-off-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>