drm/doc: Reorg drm-mm.rst

- Readjust headings - we lost one level through the extraction into a
  separate .rst file.
- Merge helper reference sections with the helper documentation - that
  split was just an artifact of the docbook toolchain sucking at too
  deep nesting levels. No such problems with sphinx.
- Move the cma helpers in with the gem documentation, since they're
  helpers to implement gem using CMA/dma memory as a backend.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-4-git-send-email-daniel.vetter@ffwll.ch