dm: replace dm_dbg() with pr_debug()

As we discussed before in ML, dm_dbg() causes undefined reference
error if #define DEBUG is added to users, but not drivers/core/util.c

We do not need this macro because we can use pr_debug() instead, and
it is pretty easy to enable it for the DM core by using ccflags-y.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

drivers/core/util.c

@@ -20,17 +20,6 @@ void dm_warn(const char *fmt, ...)
 }
 #endif
 
-#ifdef DEBUG
-void dm_dbg(const char *fmt, ...)
-{
-	va_list args;
-
-	va_start(args, fmt);
-	vprintf(fmt, args);
-	va_end(args);
-}
-#endif
-
 int list_count_items(struct list_head *head)
 {
 	struct list_head *node;