drm/ttm: revert "Export ttm_bo_get_unless_zero()"

This reverts commit 24dc64c1.

Shouldn't be needed by drivers any more.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20240723121750.2086-6-christian.koenig@amd.com