drm/ttm: rename bo manager to range manager.

The generic manager is called the range manager now, rename
the file and some internals.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-58-airlied@gmail.com