drm/tegra: Deliver syncpoint base to user space

This patch adds a separate ioctl for delivering syncpoint base number
to user space. If the syncpoint does not have an associated base, the
function returns -ENXIO.

Signed-off-by: Arto Merilainen <amerilainen@nvidia.com>
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>