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

This reverts commit 8d0b09d4.
With CONFIG_DEBUG_FS enable for non-debug build, we can enable read only
CMA debugfs nodes on all builds.

Change-Id: Ia4a0a9431e27ec908feaae54dbe622a1d985958b
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>