net/mlx5: Move CQ completion and event forwarding logic to eq.c

Since CQ tree is now per EQ, CQ completion and event forwarding became
specific implementation of EQ logic, this patch moves that logic to eq.c
and makes those functions static.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Gal Pressman <galp@mellanox.com>