drm: mm: add api for embedding struct drm_mm_node

The old api has a two-step process: First search for a suitable
free hole, then allocate from that specific hole. No user used
this to do anything clever. So drop it for the embeddable variant
of the drm_mm api (the old one retains this ability, for the time
being).

With struct drm_mm_node embedded, we cannot track allocations
anymore by checking for a NULL pointer. So keep track of this
and add a small helper drm_mm_node_allocated.

Also add a function to move allocations between different struct
drm_mm_node.

v2: Implement suggestions by Chris Wilson.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>