IB/rxe: Use rdma GID API

rxe_netdev_from_av can now be done by the core code directly from the
gid_attrs, no need for a helper in the driver.

ib_find_cached_gid_by_port can be switched to use the rdma version here as
well.

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