By Andrew Gould The Dallas Cowboys had reason to celebrate before kicking off their Week 15 game against the Buffalo Bills. During Sunday’s early-afternoon slate, the Green Bay Bay Packers and Atlanta Falcons fell to 6-8 with losses to the Tampa Bay Buccaneers and Carolina Panthers, respectively. Those outcomes locked down a playoff berth for Dallas. Dallas joins the San Francisco 49ers as the second team to clinch a postseason bid. The Cowboys will participate in their third straight postseason, and Calvin Watkins of the Dallas Morning News noted that they will pass the Packers for the most p…