IB/mlx5: Decouple MR allocation and population flows

mlx5 compatible devices have two ways of populating the MTT
table of an MKEY: using a FW command and using a UMR WQE.

A UMR is much faster, so it should be used whenever possible.
Unfortunately the code today uses UMR only if the MKEY was allocated
from the MR cache.

Fix the code to use UMR even for MKEYs that were allocated using
a FW command.

Signed-off-by: Ilya Lesokhin <ilyal@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>