net/mlx5e: Extract remaining tunnel encap code to dedicated file

Move set_encap_dests() and clean_encap_dests() to the tunnel encap
dedicated file. And rename them to mlx5e_tc_tun_encap_dests_set()
and mlx5e_tc_tun_encap_dests_unset().

No functional change in this patch. It is needed in the next patch.

Signed-off-by: Chris Mi <cmi@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>