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>