drm: remove unused #include <linux/version.h>'s

Remove unused #include <linux/version.h>('s) in
 drivers/gpu/drm/ttm/ttm_bo_util.c
 drivers/gpu/drm/ttm/ttm_bo_vm.c
 drivers/gpu/drm/ttm/ttm_tt.c

Signed-off-by: Huang Weiyi <weiyi.huang@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>