drm/ttm: minor range manager coding style clean ups

No functional changes, just removing the leftovers from the redesign.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210413135248.1266-3-christian.koenig@amd.com