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>

include/dm/util.h

@@ -15,14 +15,6 @@ static inline void dm_warn(const char *fmt, ...)
 }
 #endif
 
-#ifdef DEBUG
-void dm_dbg(const char *fmt, ...);
-#else
-static inline void dm_dbg(const char *fmt, ...)
-{
-}
-#endif
-
 struct list_head;
 
 /**