gpu: host1x: Set DMA direction only for DMA-mapped buffer objects

The DMA direction is only used by the DMA API, so there is no use in
setting it when a buffer object isn't mapped with the DMA API.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Tested-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>