drm/ttm: make ttm_bo_get internal

Prevent drivers from using this directly.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20240723121750.2086-8-christian.koenig@amd.com