net/mlx5: Bridge, extract VLAN pop code to dedicated functions

Following patches in series need to pop VLAN when packet misses on egress.
To reuse existing bridge VLAN pop handling code, extract it to dedicated
helpers mlx5_esw_bridge_pkt_reformat_vlan_pop_supported() and
mlx5_esw_bridge_pkt_reformat_vlan_pop_create().

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Paul Blakey <paulb@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>