core: make missing dpms extension non-fatal

We are not using it for anything at the moment, and it is breaking CI.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/picom.c

@@ -2131,8 +2131,7 @@ static session_t *session_init(int argc, char **argv, Display *dpy,
 	ext_info = xcb_get_extension_data(ps->c.c, &xcb_dpms_id);
 	ps->dpms_exists = ext_info && ext_info->present;
 	if (!ps->dpms_exists) {
-		log_fatal("No DPMS extension");
-		exit(1);
+		log_warn("No DPMS extension");
 	}
 
 	// Parse configuration file