IB/mlx4: Use GID attribute from ah attribute

While converting GID index from attribute to that of the HCA, GID
attribute is available from the ah_attr. Make use of GID attribute
to simplify the code and also avoid avoid GID query.

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