drm/ttm: split the mm manager init code (v2)

This will allow the driver to control the ordering here better.

Eventually the old path will be removed.

v2: add docs for new APIs.
rename new path to ttm_mem_type_manager_init/set_used(for now)

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-14-airlied@gmail.com