drm/ttm: use kvcalloc() instead of kvmalloc_array() in ttm_tt v2

simplify programming with existing functions.

v2 (chk): minimal coding style cleanup

Signed-off-by: Yang Wang <KevinYang.Wang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220421123442.1834102-1-KevinYang.Wang@amd.com
Signed-off-by: Christian König <christian.koenig@amd.com>