drm/ttm: add ttm_bo_pin()/ttm_bo_unpin() v2

As an alternative to the placement flag add a
pin count to the ttm buffer object.

v2: add dma_resv_assert_help() calls

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Link: https://patchwork.freedesktop.org/patch/391596/?series=81973&rev=1