net/mlx5: DR, Set flex parser for TNL_MPLS dynamically

Query the flex_parser id that's intended for TNL_MPLS
and use an appropriate flex parser for MPLS over UDP/GRE.

Signed-off-by: Muhammad Sammar <muhammads@nvidia.com>
Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>