drm/ttm: provide a driver-led init path for range mm manager. (v2)

This lets the generic range mm manager be initialised by the driver.

v2: add docs.
rename api to range_man_init 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/20200804025632.3868079-15-airlied@gmail.com