drm/ttm: some cleanups

Unexport ttm_check_under_lowerlimit.
Make ttm_bo_acc_size static and unexport it.
Remove ttm_get_kernel_zone_memory_size.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/390515/