Revert "mm: cma: make writeable CMA debugfs optional"

This reverts commit 099c0c9a.
CONFIG_CMA_DEBUGFS is only enabled in debug builds on 5.4+
and the purpose of this commit is to disallow CMA allocations
through debugfs on production builds. Since, this is no longer
possible, revert this change.

Change-Id: If1f322533660bfc7609fb09a5b13ff2ecbbe3135
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>