RDMA/mlx5: Allow CQ creation without attached EQs

The traditional DevX CQ creation flow goes through mlx5_core_create_cq()
which checks that the given EQN corresponds to an existing EQ and attaches
a devx handler to the EQN for the CQ.

In some cases the EQ will not be a kernel EQ, but will be controlled by
modify CQ, don't block creating these just because the EQN can't be found
in the kernel.

Link: https://lore.kernel.org/r/20210211085549.1277674-1-leon@kernel.org


Signed-off-by: Tal Gilboa <talgi@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>