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: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>