drm/tegra: falcon: Set DMACTX field on DMA transactions

The DMACTX field determines which context, as specified in the
TRANSCFG register, is used. While during boot it doesn't matter
which is used, later on it matters and this value is reused by
the firmware.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>