meminfo: make dmabuf_dump dependency on debugfs optional

dmabuf_dump had a hard dependency on debugfs to gather the initial list
of buffers for all proceses, make it optional.

Change-Id: I1d271297c0ad6124b321a1ee8aa01d3e88ca9fed
Signed-off-by: Erick Reyes <erickreyes@google.com>