RDMA/mlx5: Reduce amount of duplication in QP destroy

Delete both PD argument and checks if udata was provided, in favour
of unified destroy QP functions.

Link: https://lore.kernel.org/r/20200427154636.381474-30-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>