drm/ttm: add dma_resv_assert_held() calls to vmap/vunmap

Let's make sure nobody is calling those functions without holding the
appropriate locks.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220715111533.467012-2-christian.koenig@amd.com