net/mlx5: Refactor calculation of required completion vectors

Move the calculation to a separate function. We will add more
functionality to it in a follow up patch.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>