net/mlx5: Decrease default mr cache size

Delete initialization of high order entries in mr cache to decrease initial
memory footprint. When required, the administrator can populate the
entries with memory keys via the /sys interface.

This approach is very helpful to significantly reduce the per HW function
memory footprint in virtualization environments such as SRIOV.

Fixes: 9603b61d

 ("mlx5: Move pci device handling from mlx5_ib to mlx5_core")
Signed-off-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reported-by: Shalom Toledo <shalomt@mellanox.com>
Acked-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>