drm/ttm: flip over the range manager to self allocated nodes

Start with the range manager to make the resource object the base
class for the allocated nodes.

While at it cleanup a lot of the code around that.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210602100914.46246-2-christian.koenig@amd.com