drm/doc: Move the vma offset manager to the right spot

Currently it's sitting in the mode setting helper section, which isn't
quite right. Looks much better in the memory management section next
to TTM and GEM.

Cc: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>