RDMA/mlx5: Unfold create RMP function

There is no need to perform create_rmp in two separate function, while
one of them uses stack as a placeholder for data while other allocates
it dynamically. Combine those two functions to one instead of two.

Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>