drm/doc: Some polish for shmem helpers

- Move the shmem helper section to the drm-mm.rst file, next to the
  vram helpers. Makes a lot more sense there with the now wider scope.
  Also, that's where the all the other backing storage stuff resides.
  It's just the framebuffer helpers that should be in the kms helper
  section.

- Try to clarify which functiosn are for implementing
  drm_gem_object_funcs, and which for drivers to call directly. At
  least one driver screwed that up a bit.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200511093554.211493-4-daniel.vetter@ffwll.ch