gpu: host1x: Initialize syncval in channel_submit()

During the refactoring of channel_submit(), assignment of syncval was
moved but it is also used in channel_submit(). Add this assignment back
to channel_submit() as well.

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