RDMA/mlx5: Separate to user/kernel create QP flows

The kernel and user create QP flows have very little common code,
separate them to simplify the future work of creating per-type
create_*_qp() functions.

Link: https://lore.kernel.org/r/20200427154636.381474-29-leon@kernel.org
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>