RDMA/mlx5: Delete device resource mutex that didn't protect anything

RDMA/mlx5: Delete device resource mutex that didn't protect anything

The dev->devr.mutex was intended to protect GSI QP pointer change in the
struct mlx5_ib_port_resources when it is accessed from the
pkey_change_work. However that pointer isn't changed during the runtime
and once IB/core adds MAD, it stays stable.

Link: https://lore.kernel.org/r/6e338c561033df20d92e1371fc6a7a0d93aad945.1627040189.git.leonro@nvidia.com
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>