It’s a contraction of the expression “mettre à mort” which means to execute or more literally putting to death.

The music unfortunately makes it hard to hear distinctly, but I rather hear “les keufs, c’est mort !”

C’est mort is a fairly recent phrase (maybe 10/15 years or so ?), used rather by young people, and it means “it’s over”, “game over”, “give it up”, “won’t happen”, “forget it”. See here.

Alors, tes examens, ça va aller ?

-C’est mort, j’ai maths demain, et j’ai encore rien étudié.

Tu peux venir faire du shopping avec nous demain ?

-C’est mort, je dois travailler

Pour le match de demain, c’est mort: je me suis blessé.

So in the scene you refer to, the guy on the moto warns the gang that the cops are coming and calls them to give up the plan they had in mind and run away.