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 <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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