RDMA/mlx5: Cleanup DEVX initialization flow

Move DEVX initialization and cleanup flows to the devx.c instead of having
almost empty functions in main.c

Link: https://lore.kernel.org/r/20200702081809.423482-6-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>