IB/mlx5: Use mlx5 core to create/destroy a DEVX DCT

To prevent a hardware memory leak when a DEVX DCT object is destroyed
without calling DRAIN DCT before, (e.g. under cleanup flow), need to
manage its creation and destruction via mlx5 core.

In that case the DRAIN DCT command will be called and only once that it
will be completed the DESTROY DCT command will be called.  Otherwise, the
DESTROY DCT may fail and a hardware leak may occur.

As of that change the DRAIN DCT command should not be exposed any more
from DEVX, it's managed internally by the driver to work as expected by
the device specification.

Fixes: 7efce369 ("IB/mlx5: Add obj create and destroy functionality")
Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Reviewed-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>