At the moment Real Madrid is 8 points ahead of Barcelona with 10 matches to go. There's a total of 30 points in those 10 matches, that's enough points for Barcelona to take Real Madrid if they slip up. Let the current standing not fool you, if Madrid doesn't take the league seriously now they'll lose it.
The fact that they have Man City doesn't mean they'll pay less attention to the league. Barcelona won the league last season and Madrid won't want them to win the league back to back, that would mean going 2 seasons without winning the league.

Madrid is a big team, the top 2 biggest teams in the world. They're not a team to sacrifice one trophy for the other. They want to win everything. They'll take the league as seriously as the Champions League and if you think winning the league is not difficult for Madrid then you have not been paying attention.