RDMA/mlx5: Refactor DV create flow

Move part of the code that get the destinations into function so the code
will be more readable.  In addition change the variables definition to be
in reversed christmas tree.

Link: https://lore.kernel.org/r/20200504053012.270689-4-leon@kernel.org
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Mark Zhang <markz@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>