IB/mlx5: Don't expose MR cache in switchdev mode

When enabling many VFs and switching to switchdev mode, the total amount
of mkeys we try to allocate when loading representors is very large and
may cause timeouts on allocations, the same issues was observed on VFs
and we employ the same fix that was done for them. We avoid allocating
the full MR cache on load but still allow it to be manipulated once the
IB device is loaded.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>